Top definition
The instant where a man's penis become erect unprovoked. In other words, a random split second boner.
"I was sitting in class today and had a bone flash, it was so bad that I couldn't get up and walk around without being noticed. "

Or

Bob: "I just had a bone flash."
John: "WTF? Theres no girls in here."
Bob: "I know. Either its a bone flash or I'm gay."
-silence
Bob:
by Dr. McAwesomeBerg October 24, 2009
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