Green’s Dictionary of Slang

wonderful, first-rate, exceptional.

1968		I. Reed Free-Lance Pallbearers 3: The Nazarene Bishops are a bunch of drop-dead egalitarians crying into their billfolds.
1980		Maledicta IV:2 (Winter) 227: Camp strives for the drop-dead wit of Dorothy Parker [...] Kenneth Tynan.
1997		N. Barlay Curvy Lovebox 170: You ah two drop dead babes.
2003		T. Dorsey Stingray Shuffle 30: He had landed a drop-dead attorney in a serious pantsuit and glasses.