When someone in loose trousers bends forwards and the arse crack rises above for all the world to see.
Common with builders, council workers, plumers and general overweight people.
"My plumber was fixing my pipes and his swetty builder's bum kepted staring me in the face."
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).