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When a penis gets hard from merely being in the same vicinity as a woman, even when there is no sexual cue whatsoever.

Similar to the sensors in sci-fi movies getting set off when another space ship comes into range.
She gave her boyfriend a hello hug at the restaurant. He covertly put her hand on his hard on. She laughed and said "proximity alert!"
by Vikyliz December 31, 2011
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