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1.
another name for a penis.

background: While playing Marvel vs. Capcom 2, Iron Man's special move is called the Proton Cannon and a stream is emitted from a large gun. Protein Cannon seemed appropriate.
Prepare yourself, your girlfriend is about to be blasted by my protein cannon.
by Random Villain April 28, 2004
 
2.
A cannon full of protein
Look, that cannon is full of protein!
by Fetch-bitch November 03, 2004