Definitions by Fatboi AKA "Wetback" n proud

Slip of the thumbs 

When you are pleasuring your girl (i.e. wife, girl-friend, female friend with privileges, local slut, myspace whore.....) and your thumb "accidentally" slips into her asshole instead of her vagina.

Sorry it was a Slip of the thumbs its not like you havent had anything bigger in there.

Slip of the thumbs by Fatboi AKA "Wetback" n proud February 16, 2009