Green’s Dictionary of Slang

dead set adj.

[dead adv. + SE set, positioned]

1. [mid-19C+] fully committed.

2. [1940s+] (Aus./US) superlative, in both positive and negative uses.

In derivatives

dead-setter (n.)

[mid-19C] a committed individual, irrespective of moral standpoint.

In phrases

dead set against (adj.)

[early 19C+] totally hostile towards.

dead set on (adj.)

[mid-19C+] fascinated by, obsessed with, in love with, determined.