Excellent, of high quality. Derives from dance music, especially house, in which a tune that rocks the dancefloor is referred to as being "banging".
"That tune is banging!", "That jacket is banging!"
Much like an outie belly button; 'outie-bum' is where a portion of the rectum actually protrudes outside the anus.
When she bent over, all I noticed was her outie-bum.
