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When the scrotum becomes so tight that it pushes up the flaccid penis, causing it to resemble a semi.

May occur under conditions such as cold temperature, strenuous physical activity, or extreme anxiousness.
Josh: Dude your dick is sticking out in your sweatpants, do you have a semi?

Drew: Nah bro, I've got push-up balls.
by Manmess November 23, 2013