The line comes from Half-Life, released by Valve in 1998, where HECU marines are the military enemies sent into Black Mesa and bark orders and taunts at Gordon Freeman. Their short radio style shouts, similar to an enemy spotted call in later shooters, became easy to rip and reuse as standalone clips. As Half-Life memes and source engine edits grew online, this particular taunt survived because it works without context, sounding defiant in any fight, argument or gaming clip where someone refuses to back down.