The action of thumbing a person either in their anal cavity, or the vaginal cavity.
"Mike R: Haha look, it looks like i've been thumbing Sally when she's on her blob.
Mike C: That's not what it looks like...
Mike R: DUDE...
Mike C: Sally got Hitch hiked.
Mike R: HITCH HIKED!?
Mike C: It's when you thumb some one, instead of fingering them.