Top Definition
It's when you grab the skin between your hanging balls and your cock and then tighten your grip all the way up to your balls forcing them to really stick out, this makes your sac look veiny and bumpy like the top of a brain.
I asked my girl to examine a spot in my testes, when she came in for a closer look, I tightened up my grip and hit her in the eye with the brainer.
by allen powell January 16, 2006
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