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 29, 2008
1 more definition
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.
by LCow October 06, 2008