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When you have a massive boner in a very uncomfortable situation and your erection needs to be tucked into your waist line pointing up to your stomach to conceal it.
Mark's teacher asked him to write on the black board so he had to apply the tuck rule to hide his boner from the whole class.

Hector was flogging the dolphin to soft core porn when his doorbell rang. Girl scouts were selling cookies.. he had to apply the tuck rule so they didn't think he was a diddler.
by theDA11 January 20, 2012