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1.
The act of sliding one's penis in between the back of a bent, sweaty, knee.
This bitch gave me the craziest bumknee after her workout...Amazing!
by Chris "Backstreet" Massaroni April 10, 2011
 
2.
When your knee is injured and you remember the times before the injury.
Most Common:

I would have made pro if it wasn't for my bum knee.
by wordzilla February 27, 2011