The phrase is an older American idiom expressing amazement, and this version comes from Team Fortress 2, the multiplayer shooter Valve released in 2007. Its classes were given extensive voice lines for kills, wins and unexpected events, and players clipped many of those short reactions for use outside matches. This one spread through gaming edits and soundboards because the wording is widely understood even without game context. It remains useful as a softer alternative to shocked reactions, signalling disbelief and amusement rather than frustration, which keeps it working across many different clips.