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To attack someone from behind (whether it being a kick, punch, or headbutt) resulting in damage to the person's testicular region. A very painful and unexpected experience.
Reginald called me a bad word - I am so going to backnut him.
by Inder-House August 28, 2010
everybody has back nuts
trevor got hit in the back nuts
your freezing your back nuts off
give me a back nut
by tre tre stacy January 16, 2009