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2.
When a guy gets hard just from light physical contact with or just being around a person that he is crushing on. It happens without warning and can sometimes create an awkward situation if the other person notices.
Jimmy: Hey Daniel! Me and Jessica are having an arm wrestling contest. Do you want in?
Daniel: Nah, with Jessica there I'll just get another Accidental Boner.
Jimmy: Jessica is pretty hot. I understand.
by TheMinecraftPlaya June 10, 2013
 
1.
Unintentional wood at an awkward moment.
Boy - So I was sitting in church and this hot chick stood up and I totally got an accidental boner.
Girl - I know right????
by "Meat"ball June 18, 2009