〜にきまっている

Meaning

Expresses absolute conviction that something is certainly so, even without presenting solid grounds. As in 「無理にきまっている」, a casual expression stating the speaker's assertive feeling.

Examples

そんなうまい話は、嘘にきまっている A deal that good is surely a lie.
練習もしないで勝てるわけがない。負けるにきまっている There's no way to win without practicing. He's bound to lose.