The phrase comes from Brazilian Portuguese, where 'num sei' is a casual spoken shortening of 'não sei', meaning 'I don't know'. Online, short spoken phrases like this travel well because they can be reused as reactions without needing a full scene or explanation. This clip is shared in meme edits, reaction videos and messaging chats, where its relaxed, uncertain tone fits moments of confusion, failed explanations and absurd situations. Its appeal is practical rather than tied to one famous moment: creators can drop it into almost any video and viewers recognize the feeling of giving up on explaining something.