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1.
1. a ridiculously overpowered weapon
2. a penis
"dude the hammer of dawn in gears is such a frigging rape cannon'

"oh god she bit off my rape cannon"
by Wamfox March 31, 2010
 
2.
A large, highly destructive weapon or force. Something that basically owns the hell out of anything it goes up against.

Also, something so insanely awesome, it blows everything else away.
"This gun is such a rapecannon, I killed everyone on the map with one shot".

"That band is so rapecannon, I don't know why you would ever listen to anything else".
by SpedEx November 24, 2009