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2.
a completely useless person who is wasting my air, someone who is not retarded but acts that way
grant you are a bumknuckle on the ass of society
by paul March 17, 2004
 
1.
The lower part of the spine, the coccyx.
I slipped on the ice and landed on my bum knuckle.
by farther jack June 11, 2010
 
3.
A particularly large irritating haemorrhoid.
the steak Geoff had the night before created such difficulty pushing the poo out that it created a real bum knuckle.
by Dragon Iron January 05, 2014