The art of inserting ones thumb (thumbs), into a fellow companions anal cavity, while thrusting wildly in an up and down motion. The thumb dumper must be face to face with the thumb dumpie.

A jack off buddy (from 'cum' /'come', meaning 'semen', and 'companion'); a friend that you get together with for sessions of solo or mutual masturbation
Variant of: cumpanion (from 'companion' & 'cum'); jack off buddy & wank bud & JO buddy
Similar to: fuck buddy