Green’s Dictionary of Slang

drop-dead adv.

(orig. US) extremely.

1983		Eble Campus Sl. Nov. 2: an extremely good-looking male [...] drop dead good looking.
2000		Indep. Rev. 27 Jan. 1: I still think the suit is a drop-dead glamorous garment.
2005		R. Cea No Lights, No Sirens 201: [S]he was drop-dead focused on the blazing intensity that was my eyes.

In compounds

drop-dead gorgeous (adj.)

extremely attractive.

1992		K. Lette Llama Parlour 120: I counted twelve, drop-dead gorgeous male specimens, slouched in a nonchalant row.
1997		‘Q’ Deadmeat 139: She was drop-dead gorgeous, nice face, good bone structure, she could have been a model.
2000		Indep. Rev. 14 Feb. 19: While not conventionally drop-dead gorgeous, she lights up the screen with her presence.
2001		Pami ‘A Walk on the Wild Side’ 🌐 James looked up into a set of black eyes set in what was undeniably a drop-dead fuck-me gorgeous redhead.
2011		J.J. DeCeglie Drawing Dead [ebook] A drop-dead gorgeous smile on her maw.
2013		K. Sampson Killing Pool 70: She is absolutely drop-dead beautiful.
2017		D. Winslow The Force [ebook] A drop-dead gorgeous young woman.
2019		C. Hammer Silver [ebook] ‘That looker from down in the bush? Drop-dead gorgeous’.