when you think someone to be lying, you rub or stroke your chin, as if stroking an imaginary beard, and say "Jimmy Chin".
This is taken from the Bristish sports presenter Jimmy Hill, who had a large chin. In some places people will say "Jimmy Hill!" when making a Jimmy Chin gesture.
"I came to school in a helicopter today."
"Jimmy Chin!"
by The Cuntsaw2 February 18, 2008
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