Green’s Dictionary of Slang

1. (orig. US) exhausted, unconscious, asleep, lying down, resting.

1945		D. Bolster Roll On My Twelve 96: ‘Oh damn exercise,’ came from one chair, ‘I’m flaked out. Chrissake leave us in peace!’.
1956		Mad mag. Spring 29: While flaked out on my emerald green ‘Hide-a-bed’ [etc.].
1959		G. Slatter Gun in My Hand 186: I can’t find them. Probably snuck down in some corner possie. Or flaked out under the table more likely.
1960		G. Swarthout Where the Boys Are 34: I was flaked out from no sleep.
1971		T. Thackrey Thief 372: Flaked out in a chair, looking at the TV.
1985		B. Humphries Traveller’s Tool 61: See if there isn’t a strange sheilah flaked out on the back seat with her scanties round her ankles.
1989		C. Hiaasen Skin Tight 87: He’s on tubes in a nursing home, all flaked out.
2000		F. Mac Anna Cartoon City 108: Lucy turned him down saying that she too was pretty flaked out from a hard day.
2005		P. Howard Curious Incident of the Dog in the Nightdress 89: I’m flaked out on the bed.

2. (US campus) drunk.

1959

L.P. Boone ‘Gator Sl.’ AS XXXIV:2 156: They may later be [...] flaked out; or they have tied one on.