Dead set

Pronunciation	/ded set/ (デッド・セット)
English Definition	Absolutely true, genuine, or certain — used both as an intensifier and as an exclamation of disbelief/emphasis ('dead-set, that's amazing!').
Japanese Translation	本当に、間違いなく（強調表現）
Japanese Explanation	『本当に』『間違いなく』という意味の強調表現。驚きの表現としても使われる。

Origin

'Dead' used as an intensifier (as in 'dead certain') + 'set' (fixed/settled).