A boner you get from looking at an under-age girl.
I was watching Full House when all of the sudden Michelle Tanner gave me an illegal boner.
by MR BOYER October 30, 2008
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When you get an erection from an underage girl.
I went to the library and had an illegal boner the whole time because of all the high school girls in their short plaid skirts.

Appropriate response:

Nice.
by LCow October 6, 2008
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