Pleb comes from plebeian, the term for the common people of ancient Rome as distinct from the patrician elite. Over time pleb entered British slang and then gaming and internet culture as a joking label for someone ordinary, unsophisticated, or low status. This clip is taken from a dictionary pronunciation entry for the word, which is why the delivery sounds formal and detached. That mismatch helped it circulate as a reaction sound, shared in chats and edits whenever someone wants a quick, dry put-down without saying it themselves.