The Baku Baku no Mi is a Paramecia-type Devil Fruit that allows the user to eat anything (except Seastone). The eaten items can then be merged into and manifested from the user's body or be combined to create something else. It was eaten by Wapol.
"Bakubaku" (バクバク/ばくばく,"Bakubaku"?) is a Japanese onomatopoeia for eating quickly and wildly, specifically referring to the repeated opening and closing of the mouth. It can also refer to a heart racing from surprise.
In the Viz Manga release, and the 4Kids and FUNimation dubs, it is called the Munch-Munch Fruit, the sound effects that derived from this fruit's actions.
The fruit's major strength, as demonstrated by Wapol, is that the user gains the ability to stretch open their mouths to large proportions and be able to eat things that would otherwise be unconsumable or toxic. They also have the ability to turn their bodies into weapons depending on what they had eaten, or reconfigure and merge eaten objects and regurgitating them. It's not exactly stated how long the effects the user's transformed body stay with the modifications.
While the devoured matter allows the user to give a great variety of applications in battle and out, it renders the user constantly hungry, as Wapol complains about, though it does benefit the user to eat a seemingly unlimited amount of food without feeling strained. "Chewy" foods, like rubber, are slightly hard to swallow for the user. Also, the user can only use the objects they have eaten in the previous 24 hours, anything past that time would not be usable anymore. Other than that, the user is also affected by the standard Devil Fruit weakness.
The fruit's bestowed powers, as used by Wapol, have been used mostly to help Wapol engorge himself on various things. These include various things from knives to the houses of Wapol's former subjects. Due to this, Wapol can become at times a complete nuisance to those around him with vast amounts of property damage, just so he can fulfill his appetite. Wapol can also use the fruit's powers for combat purposes by either swallowing his opponent whole or by using the fruit's transformation powers to aid him. He also managed to survive an avalanche and save his two henchmen by swallowing the snow masses and keeping his subordinates safe in his mouth.
After being defeated by Luffy and becoming a homeless bum, Wapol was able to come up with more beneficial applications of the fruit's powers and serve more productive purposes. Using his Baku Baku Factory technique, Wapol was able to create some toys to sell for a livelihood. In return, he was able create a new industry for him to rule over.
Yukigesho (雪化粧,Yukigeshō?, literally meaning "Snow-Covered"): Wapol hides himself in the snowy landscape of Drum Island and ambushes any trespassers by jumping out with his jaws wide open, ready to swallow said trespasser. He can also hide Chess and Kuromarimo as backup, should this attack fail. In the Viz English Manga and FUNimation dub, this technique is called Snow Powder Makeup.
Baku Baku Shock (バクバク食,Baku Baku Shokku?, literally meaning "Chomp Chomp Food"): After eating something, Wapol can mutate the object or material into his body such as turning his arm into a cannon. This was first seen being used by Wapol to turn himself into a house with cannons for arms. The various forms seen so far Wapol has changed into using this technique are a large form consisting of various weapons, a house with cannons for arms, a wooden form with a long branch for a nose and tree coming out of its head, a form with a lamp for a head complete with a burning candle, and a park bench. This is called Munch-Munch Mutation in the Viz Manga and 4Kids dub and Munch-Munch Shock in the FUNimation dub.
Wapol House (ワポルハウス,Waporu Hausu?): After one "butter sauteed cannon", one "cannon extra rare", a "cannonball and gunpowder salad", and one "grilled house", Wapol absorbs them into his body and transforms into a house complete with doors, chimney, and windows included. His arms are also transformed into cannons, and the chimney can also act as a cannon. This form was first seen being used by Wapol to fight against Luffy and Chopper at his castle. The name may be a pun on the restaurant chain "Waffle House".
Bero Cannon (ベロ大砲,Bero Kyanon?, literally meaning "Tongue Cannon"): Wapol transforms his tongue into a cannon and fires a cannonball at his opponent. This was first seen being used to fire at Luffy when he grabbed Wapol's jaw tightly with his hand. In the Viz Manga and 4Kids and FUNimation dubs, this is called Tongue Cannon.
Ningen Heiki (人間兵器,Ningen Heki?, literally meaning "Human Weapon"): Described by Wapol as his most powerful transformation, after eating all the weapons in Drum Castle's armory, Wapol uses Baku Baku Shock to transform his entire body into weaponry. First seen being used to defeat and punish Dalton in a flashback, Wapol tried to use it against Luffy during their battle, but was unable to due to Nami having stolen the armory key from him.
Baku Baku Factory (バクバク工場,Baku Baku Fakutorī?, literally meaning "Chomp Chomp Factory"): A technique in which Wapol combines two or more different things he has swallowed to create something else. This was first seen being used when Wapol fused Chess and Kuromarimo together to create Chessmarimo. The way Wapol fused them together into a form that talks with two voices simultaneously resembles fusions found in Dragon Ball. After being defeated by Luffy, Wapol used this technique often in order to create toys as a means of livelihood. A by-product found in the toys created by this technique, that was discovered by scientists, is a new type of steel, hinting that this fruit may also fuse undiscovered minerals. This steel was dubbed by the scientists as Wapometal. This is called Munch-Munch Factory in the Viz Manga and 4Kids and FUNimation dubs.
Slim-up Wapol (スリムアップワポル,Surimuappu Waporu?): Wapol swallows most of his body until only his lower jaw is left. He then rearranges his bone structure using his powers. When he's done, he pops out and reveals himself as a skinnier and taller version of himself. This was first seen being used in order to chase Nami better. This is called Wapol Slim-Down in the Viz Manga and Lean and Mean in the 4Kids and FUNimation dubs.
Musshuru Wapol Cannon (ムッシュールワポルキャノン,Musshūru Waporu Kyanon?): A fusion of Wapol with his brother, Musshuru, and the Royal Drum Crown 7-Shot Bliking Cannon. With Wapol as the dominant personality, he could still utilize his brother's Devil Fruit powers and release the poisonous spore bomb via a timer on his chest. The Bliking Cannon is attached to his back, pointing upwards, with the Fatal Bomb loaded inside. He also wielded four arms, two of his own with axes and two of his brothers. This form was seen being used by Wapol so as to wipe out the entire Drum Island population.
A fan asked Oda what would happen if Wapol ate Luffy, Zoro, Nami, Usopp, Sanji, Chopper, and Robin and combined them together like Chessmarimo. Oda responded to the fan's request by drawing the combination for the fan and naming it "Gargon". This amalgamation possessed Luffy's Straw Hat, Chopper's antlers, Nami's hair situated on its shoulders, Zoro's swords and haramaki, Sanji's eyebrow situated on top like a strand of hair, Robin's fringe, and Usopp's nose situated on its left shoulder.
Though the natural law of Devil Fruits dictates that only one person can only have one Devil Fruit power at any one given time, the fact that Wapol's Baku Baku no Mi powers can fuse two people or more together into a being with combined abilities might be a loop hole of sorts in the law. This was addressed in the ninth One Piece movie where Wapol ate his older brother and became a fused being with two Devil Fruit abilities. This aspect of the fruit's power is non-canon, and when Oda was asked by a fan about it in an SBS if Wapol would gain Luffy's rubber powers by eating him, Oda only responded by saying he will get food poisoning.
The way the powers of the Baku Baku no Mi works is similar to the old phrase 'You are what you eat'.
The Baku Baku no Mi's ability to consume undigestible materials such as wood and other items is similar to the disease pica where victims are forced to have sudden cravings and eat food that would be classified as inedible by a normal person.