Pen Cap Effect - When a male of the human species has an erection while laying down and tries to roll over from ones back to his stomach and is stopped by ones erection either way he rolls thus resembles the way of a "Pen Cap" would roll being stopped by the part of the pen cap in which you use to hold your pen to a sheet of paper or the pocket of your shirt.
Jon: Man I couldn't get comfy last night.

Kylie: Why not?

Jon: Well I wanted to lay on my stomach but every time I tried to roll over I would get the Pen Cap Effect and would be afraid it would break or something.

Kylie: Oh dear...
by Kornaxx19 May 06, 2012

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