An action performed in Modern Warfare 2; used when talking about somebody who repeatedly uses the tactical knife attachment to perform kills. It's frequently paired with the perk Commando. This is a tactic commonly used by players deemed as a hiker.
Guy 1: Tac knifing is for n00bs!
Guy 2: What are you talking about? You were tac knifing yesterday when you called in that nuke at Sub Base!
Guy 1: I was tac knifing your mom last night.
Guy 2: ...
Video game lingo often used when playing a first-person-shooter online, meaning to run around the map whilst making many kills by repeating the same tactic(s); the opposite of being a camper.
Greg: Alright, so I was playing Modern Warfare 2, and I tried camping at Crash.
Greg: This guy kept running around the map tac knifing everybody. He even called in a nuke!
Fred: What a freakin' hiker!