When you blow your load in a chick's knee pit after a good knee fuck, and then have the girl jump on an exercise bike. The squishy sound of sticky cum in her knee pit is the Philadelphia Plunger

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