When your girlfriend won't have sex with you; and you precede to pleasure yourself manually in full vision of her, fully flaunting your engorged member.
This may be done
(1) out of spite,
(2) in an attempt to induce her into sexual activity,
(3) or just simply to get a nut off.
Guy 1: Hey dude, did you get any pussy last night?
Guy 2: No damnit, my girl was tired so I showboated
Guy 1: What a bitch! Always making you turn to showboating.