Top Definition
A fraction of an inch, because men often exaggerate the lengths of their members.

If, on average, a man with a five inch penis claims it's six inches long, then a penis inch is 5/6 of an inch.
The window is forty penis inches wide, but really it's only three feet.
by Johnny Rocker October 01, 2009
A standard of measurement with no basis in reality. The phrase derives from men who exaggerate the size of their penis.
I think I can fit in that parking spot.

Seriously? What, are you measuring in penis inches?
by rickbrunernyc August 06, 2009
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