Green’s Dictionary of Slang

a betrayal of something, usu. a secret; usu. as dead give-away; also attrib.

1872		E. Crapsey Nether Side of NY 91: [R]ecovering the property, without his fellows declaring that it has been ‘a dead give-away,’ whereby they mean to say that no skill has been displayed in the matter, because the detective has been guided from the outset by the thief who stole the plunder.
1880		Colfax Chron. (Grant Parish, LA) 21 Aug. 2/3: You saw what a give-away his looks are [...] You’d think a jeweler would send for a policeman.
1882		W.A. Baillie-Grohman Camps in the Rockies 14: The dead ‘give away’ that was in store for the bad man.
1882		Nat. Police Gaz. (NY) 25 Nov. 6/4: [A] ‘dead give away’ conversation between them was overheard .
1899		Ade Fables in Sl. (1902) 196: The last Chapter is a Give-Away. It condenses the whole Plot and dishes up the Conclusion.
1912		A. Berkman Prison Memoirs of an Anarchist (1926) 197: You’re a dead give-away.
1918		E. Pound letter 15 Nov. in Paige (1971) 140: Seagan was quite intelligent when she brought him from France, but the months in Ireland have ruined his mind and left him, as might be expected at his age, doomed to political futilities. He is a walking give-away of the real state of feeling there.
1928		J. March Wild Party 14: The bath was a horrible give-away. The floor was dirty: The towels were grey.
1940		O. Strange Sudden Takes the Trail 95: Don’t forget to blot the brand o’ that hoss you took in exchange for yore own; she’s a dead giveaway.
1953		F. Brown Madball (2019) 80: ‘Use your head, Mack, that’d be a dead giveaway’.
1961		Galton & Simpson ‘Economy Drive’ Hancock’s Half-Hour [TV script] It’s a dead give-away for burglars, that is.
1971		R.D. Pharr S.R.O. (1998) 466: ‘He don’t ask to cash no check and so that’s a natural giveaway’.
1977		G.F. Newman Villain’s Tale 16: And then the containers themselves, they’d be a dead give-away if he were stopped with those in the car.
1981		J. Ellroy Brown’s Requiem 202: It didn’t take me long to spot the caddy shack [...] the carelessly dressed men coming out of it were a dead giveaway.
1996		R. Doyle Woman Who Walked Into Doors 128: I look modern; you’d never think it was long, long ago. The flares on Charlo’s trousers are the big give-away. And the hairstyles.
2004		‘H.U. Quickwit’ Plagiarizm 33: This is an absolute dead giveaway that the paper has been lifted.