It's a nickname of XXXTENTACION, it was created by himself during a video where he showed his muscles while boxing with one of his friends.
It means that his muscles are so big that he can use them like tentacles.
(The tentacles of polyps are known for beign really strong.)
A small piece of information. Derived from the word ken, used often in the scottish language and is synonymous with knowledge.
Person 1: "Hey I don't get this shit. How do you solve this problem?"
Person 2: "I got that one. Give me some kenlets on this assignment and I'll help you w/ that one."