builder's bottom/bum

It is a term that is used when someone in loose trousers bends forwards and the bum crack rises above for all the world to see.
Common with builders, council workers, plumbers and generally overweight people.
-"My plumber was fixing my pipes and this huge builder's bum kept staring me in the face."

In fact, I have seen this happening with young men or women when they bend down and it has probably happened to all of us when we bend down: