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1.
When a lady gets so old that her breasts sag all the way down to her knees, they then stop being breasts as such and become "knee bangers". Another tell tale sign is when you shout, "Show us your tits!", they must lift their skirt in order to do so.
(Can also be spelt as one word - "kneebangers")

"Those knee bangers make her look like she has four arms!"
"Yeah you're happy know because they're big and firm, just wait until your older... when she turns sharply, you'll get some knee bangers to the face..."

by Gr8 January 15, 2008
 
2.
A descriptive noun used to explain the length of mans very large penis.
You see that sully in the shower with the knee banger. Motha Fucka is hung like a walrus.
by Cornelius of Gump October 22, 2012
 
3.
A situation or circumstance causing one's knees to shake nervously and/or uncontrollably.
"Wow! That confrontation with your boss must have been a real knee banger!"
by Susan Beamer September 20, 2007