(1)When you are/are not in a relationship. and have a person you fuck on the side. Side duffs manly are just people who you don't really care about, but just only to fuck.

(2) Someone who likes/loves you a lot and buys you stuff and is always promised sex, but never get any in return.

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