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
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