the process of masturbating immediately after your hand/arm has become numb either naturally, from a shot, or from sitting on your hand on purpose. Its main purpose is to make it feel like you're not jacking off by yourself in a dark room watching a porno that you've already seen dozens of times.

Also mentioned on the Comedy Central tv show, "Ugly Americans"

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