Monsieur is simply the French word for mister, and as a short audio clip it circulates online mainly as a reaction and greeting sound rather than as a quotation from a particular film or show. Its appeal comes from the word itself: instantly recognisable, easy to understand even for non-French speakers, and formal enough to feel funny when applied to pets, friends or everyday situations. Because the clip is only about two seconds long, it fits neatly into memes, edits and short-form videos, where it can set a tone in a single word without needing any explanation.