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When someone uses their brain to the full, this can be both bad or good.
This can be interpretted in many ways it basically just means using your brain to your full potential.
Person 1: Hello you crazy axe wielding chimpanzee,
Person 2: God aren't you mentally erect.

Person 1: Im so pissed of your a right fucking idiot why do you always do this to me?
Person 2: Calm the fuck down mentally erect child.
by Cosa nostra is my thing! July 03, 2011
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