if something is declared in a bar whisper, it means the speaker is trying to convey something in private to another person (typically a friend at a bar), but is unfortunately too intoxicated to control how loudly they're speaking.
in other words, it's when someone's too shit-faced to keep their voice down and usually ends up proclaiming something sensitive to everyone at the bar. very loudly. this can often result in a major cock-block for the rest of one's group.
PETER: hey babe, want to head back to my place?
CHRIS: (attempting to whisper) you told me earlier you thought that chick looked like a trannie!
DRUNK CHICK: (no longer wants to go home with peter)
PETER: you cock-blocking asshole, nice fuckin' bar whisper!
DOUG: (attempting to whisper) we should get outta this club soon, my hemorrhoids are acting up.
HOT STRIPPERS: (flee the scene)
TOM: doug, man, you really need to work on that bar whisper.