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The result when you and your best friend do something immensely awesome and defining of a best friendship together. Also the result of recollecting or reminiscing of how awesome you and your best friend are.
1. "Dude, I just heard Man In The Box on the radio and got a bestfriend boner remembering that time we sang it together in Poli Sci."

2. Tim and Justin will forever remember the night they got bestfriend boners, after catching the house on fire at a party.
by Daniel Caldwell November 08, 2013