To jam your thumb in the anus and make her scream daddy till she cums

To hork or lodge the erect thumb straight into the anus of a woman. "also know as lil butt thumb"

Butt thumbing is much like butt fingering, except that the thumb is used. It tends to be more intense because of the dexteral advantage of the thumb over fingers for two main reasons:
1. The thumb does not have other fingers getting in its way
2. It is much sassier

When you're fucking a chick minding your own business and a bitch sticks her thumb up your ass, so you slap the shit out of her and throw her nasty ass to the curb.

A butt growing out of ur ass