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1.
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."
by Mr Yin January 06, 2009
 
2.
When a bird bends down and the top of her arse crack is on show
If she bends down any further she'll flash her builder's bum
by MK'chenko July 06, 2006
 
3.
a condition in construction workers where a fall onto a piece of equipment causes it to become lodged in the anus
A 31-year-old builder was left in agony from Builder's Bum, when a freak slip from a ladder thrust the handle of a shovel 20 inches up his backside.
by eisiger May 04, 2011