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.