The line comes from Team Fortress 2, Valve's team-based shooter released in 2007, where the Engineer class builds a sentry gun that enemy Spies can sap to disable it. When that happens, the Engineer calls out the attack in his distinctive Southern accent, and players quickly turned the warning into a shorthand for being caught off guard. It spread through gameplay clips, TF2 memes and short-form edits, and it remains popular because the frantic delivery instantly signals sabotage without needing any explanation.
About this sound
The Engineer from Team Fortress 2 warns that a Spy is sapping his sentry gun, a short voice line gamers use for betrayal, sabotage and surprise attack clips.
The clip is the Team Fortress 2 Engineer shouting in his thick Texan drawl that a Spy is draining his sentry gun, his voice rising in urgent alarm over just a few seconds. The delivery is loud, clipped and slightly panicked, with the mangled pronunciation making the warning instantly recognisable even to players who have never touched the game. Hearing it triggers an immediate sense of something going wrong behind your back, which is why it lands so well as a punchline for betrayal, sabotage or a plan falling apart at the worst possible moment.