Fapadapt is the act of starting to mastrubate to a new kind of thing. It could be anything from teletubbies to hentai. When you start mastrubating to a thing, you have fapadapted it. Fapadapt originates from the words fap and adopt.
Person 1: I've fapadapted hentai!
Person 2: Dude that's freckin' weird!
Person 1: Not as weird as your face!