It is when you are in a hotel room and have sex with your girlfriend while her mom sleeps in the bed right next to you.

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

When you and your girl are doing the Boner Jams Dance in a hotel room while her mom is sleeping in the other bed.