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1.
adj

it means to kill or defeat in a quickly, chaotic manner. often with over kill. used often in relation to other players online or on the internet and how you will treat them
"i'll gun-run those mother fucking sons of whore mongers" after somebody pisses you off in multiplayer
or
"I'm gonna gun-run those bitches right down to hell"
also can be used with pwn obliterate overkill etc.
by Mr. Psychic September 25, 2010
 
2.
A workout at the gym in which the person only works out their arms in order to make them look bigger; biceps, triceps, forearms
Bro, we need to get a quick gun run in and get our swell on before the club tonight!
by jackshow December 07, 2011