The clip comes from Half-Life 2, Valve's 2004 first person shooter, where distorted screams and panic sounds help sell the horror of City 17 and its alien occupation. Clips from the game were ripped and shared on forums, YouTube and soundboards in the late 2000s, then reused far outside the game. As the scream meme format grew, this specific wail became a shortcut for fake terror in edits, because players recognised it instantly and non players still understood the joke.