Juku comes from jukusuru (熟する,jukusuru?), which means "to mature" in Japanese.
Its English name "Ripe" is another term for maturing.
Strengths and Weaknesses
By simply touching an object, the user will cause it to rapidly age and decay. Using it on the ground will cause it to become a sinkhole, allowing the user to potentially avoid attacks by sinking into the ground. The fruit can also help the user get past physical obstacles by weakening them through aging, causing them to collapse or be easily broken through. This power can also be applied on living beings, causing them to grow older in an instant.
The rate of decay will continue to progress once triggered, thus leading to potential problems after the initial purpose, such as a sinkhole becoming deeper until it reaches to the ocean. Also, if the user ages a living being, this will only apply physically, as the subject will retain their original mental maturity. Any effect of maturing cannot be reversed, meaning that anything, inanimate or alive, will not be able to return to their previous state. Other than that, the fruit has the standard Devil Fruit weaknesses.
While Shinobu can employ her powers into living things in order to make them grow older, she appears to have moral reservations over this aspect of her ability, since a matured person will still retain their undeveloped mentality (should they have been a child beforehand) and there is no method to reverse the aging. When Kozuki Momonosuke ordered Shinobu to use her Devil Fruit to age him up, she refused her lord without hesitation due to having knowledge of the downsides.
Sexy Ninpo: Kare-San-Sui (熟々忍法 枯散衰,Sekushī Ninpō: Kare San Sui?, literally meaning "Mature-Mature Ninja Art: Wither-Scatter-Decline): Shinobu touches the ceiling in which she hides and makes it collapse in order to escape. If there is someone below her position in a lower floor as she performs this move, they may end up hit by the falling ceiling debris. In the VIZ translation, this is called Sexy Ninpo: Rot Garden. The technique's name may be a pun on karesansui (枯山水,karesansui? literally meaning "dry landscape"), a term for the Japanese rock garden.
This fruit is similar to the unnamed fruit eaten by Jewelry Bonney as both fruits focus on aging. Unlike this fruit, Bonney's fruit can also make the target younger, and seems to only affect living things.
This fruit's power is basically the opposite to that of the Modo Modo no Mi, as the user can make objects and creatures younger instead of older.