Bucking the mustard is basically having diarrhea so bad it's squirting yellow from your rectum like someone's squeezing a bottle of mustard and it farts towards the end. Generally followed by the sound of someone scrubbing the shit splatters off the side of the bowl.

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