When you Sonic Blastoff so hard into Rainbow's ass that both of you shoot off into a Double Rainboom.

Basically fucking the rainbow out of her.

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