rude slang To show one's bare buttocks, as to an unsuspecting audience. "BA" is an abbreviation for "bare ass." Please tell me you didn't hang a BA at the headmaster after graduation.
hang a BA
verb
See hang a BA at someone
hang a BA (at someone)
tv. to display one’s buttocks to someone in derision. (A BA is a bare ass.) He went to the window and hung a BA—just for the hell of it.