When your friend is driving your pregnant wife around, and is such a terrible driver you are worried that your baby might come out pre-shaken, ie sloshen baby syndrome.

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