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vulgar slang The penis as a source of a man's impulses or decision-making (as opposed to the brain in a one's real head). I know you're attracted to Sarah, but stop thinking with your little head long enough to consider how hooking up with my best friend would affect me. You need to stop making decisions with the little head in your pants and start using the big one on your neck.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.