Literally, having ones head up ones ass. Of course, that could be seen as crude, so a veneer of medical-esque terminology makes it seem more erudite!

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