When full it contains "the best stuff on earth" but empty, it becomes the perfect mobile urinal when stuck in your car.

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 full it contains "the best stuff on earth" but empty, it becomes the perfect mobile urinal when stuck in your car.