Gluposti comes from everyday speech in several South Slavic languages, where it means nonsense, silliness or stupid things. As a soundboard clip it works less as a translation and more as a verdict, which is why it travels well beyond speakers of the language. Short reaction sounds like this spread through meme pages, group chats and video edits because the meaning is clear from tone alone. It stays popular alongside other blunt dismissal sounds, and sits naturally next to a drawn out faaaa when a reaction needs to escalate.