Green’s Dictionary of Slang

(Aus.) definitely, certainly; also as excl.

1975		Bulletin (Sydney) 26 Apr. 44: Dead set the best bake in history they give me and I got the book for a shit pot dud.
1983		J. Byrell (con. 1959) Up the Cross 13: He was dead set correct in his judgement.
1985		R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 19: This dead set shits on cartin’ beef.
1992		R.G. Barrett White Shoes 146: A bunch of wogs tried to kill me on Friday, Les. I’m dead-set lucky to be alive .
2003		Bug (Aus.) June 🌐 Dead set, it must be hard to be a Blues coach with so much talent.
2015		N. Cummins Tales of the Honey Badger [ebook] I was dead-set shitting myself.