The sound comes from Half-Life, the 1998 first-person shooter by Valve, where the non-player scientists in the Black Mesa facility react to damage with a set of short pain grunts. Several variants were recorded, and this second version became a staple in machinima, Garry's Mod videos and later meme edits. Like the Roblox oof, it survived long after the original game because its short, recognisable human reaction fits almost any fail clip, keeping it in constant use on soundboards and in gaming edits.