Javorius Allen has had the nickname Buck since he was a sophomore in High School.
In high school, he was friends with a Tight End on the football team named Ja'Baris Little and their names would often get mixed up by other people. Allen was also a member of the varsity team despite his age, which led to him being called Young Buck, which eventually shortened to just Buck, a name he has kept ever since.
Allen is fine with either, which is a good thing, as Terrell Suggs has given the nickname Buck to Daryl Smith and knowing Suggs, he isn't going to give a respected teammates name up to a rookie.
He might give Javorius a less than savory nickname though.