The line comes from the Italian dub of The Big Lebowski, the 1998 Coen brothers comedy that became a cult classic quoted far beyond its original release. In Italian, 'Non lo sapevo' is an everyday phrase, but clipped this short and delivered this flatly it turned into a reusable reaction sound online. Italian meme pages and editors picked it up for edits where a big revelation deserves a shrug instead of shock, and its brevity keeps it popular for quick video replies and chat soundboards.