The phrase smooth like butter is a long-standing English idiom for something that moves or works with total ease. Its exact wording reached a much wider audience in 2021, when the opening line of the BTS song Butter repeated it as a confident hook. Short clips of the line then spread across TikTok, Reels and meme edits, where creators used it as a ready-made caption for anything slick or flawless. It remains popular because the meaning is instant, even without context.