Epithet

An epithet is a a name to describe a person or object. It can also be interpreted as a nickname or a title granted to a person.

Epithet in One Piece
In One Piece, there are plenty of people with epithets. Plenty of famous characters are known by these nicknames, especially pirates. Usually these titles are granted to pirates by the World Government when they are assigned bounties, to make them more well known to the world, but there are exceptions that the pirate has already chosen a nickname for himself prior to being marked down for being wanted.

Aside from pirates, there are also others who gain epithets, such as other types of criminals, Marine officers, or other significant figures.

Usually, these assigned epithets are based on the description of the person's attributes, skills, or what they simply are.

Straw Hat Pirates

 * Monkey D. Luffy: "Straw Hat" (for his prized straw hat that he received from his role model, Shanks)
 * Roronoa Zoro: "Pirate Hunter" (for his infamy of a powerful bounty hunter prior to joining the crew, and the nickname still stands)
 * Nami: "Cat Burglar" (for her sneaky thieving skills)
 * Usopp:
 * "Sogeking" (more like an alias and a split personality, this is his recorded name on the wanted poster)
 * "King of the Snipers" (the epithet for Sogeking, for his great marksmanship)
 * Sanji: "Black Leg" (for his powerful kicking martial arts along with his black pants)
 * Tony Tony Chopper: "Cotton Candy Lover" (for his sweet and innocent looks, an overlook on his true skills by the government)
 * Nico Robin:
 * "Miss All Sunday" (her codename for Baroque Works)
 * "Devil Child" (for her ability to read Poneglyph, leading to the misinterpretation that she wants to destroy the world)
 * Cutty Flam:
 * "Franky" (his new name, after discarding his original for a more convenient way of addressing him)
 * "Cyborg" (for his self-modifications into a cyborg)
 * Brook: "Humming Swordsman" (for his singing while striking with his sword)

Shichibukai
Note: Marshall D. Teach is part of this list.
 * Juracule Mihawk: "Hawk Eyes" (for his eyes are like a hawk's)
 * Sir Crocodile: "Mr. 0" (more of a codename for Baroque Works)
 * Bartholomew Kuma:
 * "Tyrant" (for his vicious tyranny of crushing everything in front of him)
 * "Pacifista" (for his participation in the Pacifista project)
 * Boa Hancock:
 * "Pirate Empress" (reason unknown)
 * "Snake Princess" (her title as ruler of the Kuja tribe)
 * Jinbei: "Knight of the Sea" (reason unknown)

Blackbeard Pirates

 * Marshall D. Teach: "Blackbeard" (for his black beard)
 * Van Auger: "The Supersonic" (for his great marksmanship and speed)
 * Jesus Burgess: "The Champion" (for his champion wrestling skills)
 * Doc Q: "Shinigami" (reason unknown)

Yonkou and Crewmates

 * Shanks: "Red-Hair" (for his red hair)
 * Rockstar: "Rookie" (for still being a rookie)
 * Edward Newgate: "Whitebeard" (for his white beard)
 * Portgas D. Ace: "Fire Fist" (for his Fire Fist technique)

The Eleven Supernovas
Note: both Monkey D. Luffy and Roronoa Zoro are part of this list.
 * Capone Bege: "Gang" (for his command over a gang of mafia-like pirates)
 * Jewelry Bonney: "Big Eater" (for her gluttony)
 * Basil Hawkins: "The Magician" (for his voodoo and fortune telling abilities)
 * Eustass Kid: "Captain" (reason unknown)
 * Scratchmen Apoo: "Roar of the Sea" (for his sound-based abilities)
 * X. Drake: "Red Flag" (reason unknown)
 * Killer: "Massacre Man" (for his responsibility of high number civilian casualties)
 * Urouge: "Mad Monk" (reason unknown)
 * Trafalgar Law: "Dark Doctor" (reason unknown)

Other Pirates

 * Gol D. Roger: "Pirate King" (for his ultimate fame)
 * Alvida: "Iron-Mace" (for her iron mace weapon)
 * Buggy: "The Clown" (for his circus theme crew)
 * Mohji: "The Lion Tamer" (for his lion taming)
 * Cabaji: "The Acrobat" (for his acrobatic skills)
 * Kuro: "of a Thousand Plans" (for his brilliant planned attacks)
 * Jango:
 * "One Two" Jango (for his catchphrase for his hypnosis)
 * "Double Crosser" (for his defection from the Black Cat Pirates)
 * Zeff: "Red-Leg" (for his kicks leave blood on his legs)
 * Don Krieg:
 * "Pirate Admiral" (for his Pirate Armada)
 * "Foul Play" (for his deceptions)
 * Pearl: "The Iron Wall" (for his Iron Wall techniques)
 * Gin: "Man-Demon" (for his cruelty and mercilessness to his opponents)
 * Arlong: "Saw-tooth" (for his saw-like tooth)
 * Yorki: "Calico" (reason unknown)
 * Dorry: "The Blue Ogre" (reason unknown)
 * Broggy: "The Red Ogre" (reason unknown)
 * Wapol: "" (for his metal-eating abilities)
 * Masira: "Salvage King" (for his salvaging skills)
 * Shoujou: "Sonar King" (for his sonar-like voice)
 * Roshio: "The Executioner" (reason unknown)
 * Bellamy:
 * "The Hyena" (for his scavenging of other pirates' treasure is similar to that of a hyena)
 * "The Big-Time Rookie" (for his so-called big bounty)
 * Sarquiss: "Big Knife" (for his "Big Knife")
 * Foxy: "The Silver Fox" (for his deceptive and cheating nature)
 * Lola: "Marriage Proposal" (for her repeatedly attempts to propose for a marriage)
 * Dias: "Devil" (reason unknown)
 * Silvers Rayleigh: "Dark King" (reason unknown)
 * Fisher Tiger: "The Adventurer" (reason unknown)
 * Kinjishi: "The Flying Pirate" (reason unknown)

Marines

 * Morgan: "Axe-Hand" (for his axe hand)
 * Fullbody:
 * "Iron Fist" (for his iron knuckles on one hand)
 * "Double Iron Fist" (for his iron knuckles on both hands)
 * Smoker: "White Chase" (for his smoke abilities)
 * Hina: "Black Cage" (for her caging abilities)
 * Sengoku: "The Buddha" (reason unknown)
 * T-Bone: "Ship Cutter" (for his ship-cutting swordsmanship)
 * Kuzan: "Blue Pheasant" (alias for being an Admiral)
 * Sakazuki: "Red Dog" (alias for being an Admiral)
 * Monkey D. Garp:
 * "The Fist" (for the use of his powerful fists)
 * "Hero of the Marines" (for cornering Gol D. Roger on multiple occasions)
 * Borsalino: "Yellow Monkey" (alias for being an Admiral)

Other Epithets

 * Montblanc Norland: "Liar" (for a biased view of his life)
 * Calgara: "Demon Warrior" (for his berserk skills)
 * Eneru: "God" (his status over Skypiea)
 * Monkey D. Dragon: "Revolutionary" (for his leadership in the Revolutionaries)
 * Ryuuma: "The King" (for his skills as a swordsman)
 * Oz:
 * "The Devil" (for his extreme brutality)
 * "The Continent-Puller" (for his immense strength)
 * Jigorou: "of the Wind" (reason unknown)

Anime Only

 * Billy: "The Bandit" (reason unknown)
 * Dick: "Mountain Whale" (reason unknown)
 * Woonan: "Great Gold Pirate" (for his legendary treasure)
 * Erik: "The Whirlwind" (for his wind cutting nails)
 * Bigalo: "The Hanged Man" (reason unknown)
 * Puzzle: "Phoenix" (for his ability to keep standing up to fight)