昭和風の先生と少女のちょっと大人な会話

What does “に決まってる” (ni kimatteru) mean in Japanese?

に決まってる turns a personal opinion into a flat statement of fact — “it’s obviously true,” “no doubt about it.” Attach it to almost anything: 無理に決まってる (“that’s obviously impossible”), 嘘に決まってる (“that’s obviously a lie”). It’s more emphatic than just saying no — it shuts down the question entirely, the tone somewhere between “duh” and “trust me, I know.”

The full, polite form is に決まっている; drop the い for the casual に決まってる you’ll actually hear in conversation. It’s built on 決まる (“to be decided, to be fixed”) — so it literally reads “it’s decided/settled that...”, which is why it lands as blunt certainty rather than a hedge. In this Shōwa-style teacher-student scene, a request gets ruled out with exactly this force: no room left for debate.