To un-intentionally show you bum. Usually only the top part is revealed when bending down. Or if you’re a builder of some description you will already be wearing loosely fitting pants that assist in revealing your bum crack.
I was walking along the street when I saw two moons looking at me. The fattest builder was showing every one his bum crack. To which I shouted out say no to crack!
An inch long crevice found on the derriere, that has escaped the modest covering from trousers.
"Cor, look at her bumcrack, the dirty slapper! You could park a car in that!"