When you have a sudden increase in blood flow to the penis from sexually arousing scenarios which causes the penis to puncture a hole through the article of clothing in which the stiffy is concealed.
Pop a stiffy mate.
by Chode Buster April 29, 2011
A way of stating that the male penis is soon to ejaculate. Otherwise known as a boner.
I Pop a Stiffy everytime I get to walk behind Payton
by Blevinz11z March 02, 2012