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refers to the day of the week, followed by Thursday, in which guys go to the gym before going out to solely work on the arms (firearms) and then proceed to kick off the weekend of partying with shotgunning a beer thereafter (firearm)... a total frat move
"Firearm Friday is tommorow, so we have to do bi's and tri's at the gym for the ladies right before shotgunning those Budweisers"
by Brandford MacArthur January 30, 2014