The act of somebody placing a hand on the opposing shoulder, and then leaving it there, as if fondling their own shoulder blade. People usually shoulder-grope themselves while discussing their accomplishments or positive traits (often with a sentence beginning "I'm pretty much the best..."), and it is therefore regarded as a narcissistic celebration of one's own awesomeness. On occasion, both hands are employed, giving the shoulder-groper the appearance of somebody hugging themselves because they have no friends.

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