Green’s Dictionary of Slang

no-shit adj.

(US) genuine; also as adv.

1945		in T. Shibutani Derelicts of Company K (1978) 208: That’s no shit. He’s really a regular guy.
1958		Kerouac letter 13 Sept. in Charters II (1999) 134: I’m genuinely actually no shit afraid of walking down North Beach and all of a sudden some old buddy rushes up with pockets loaded with shit and the cops swoop in.
1966		B. Jackson Get Your Ass in the Water (1974) 118: On the time of your trial she’ll be there and that’s no shit, / not to do you any good but to see how much time you gonna get.
1970		D. Mitchell Thumb Tripping (1971) 132: Jacky’s husky, no-shit talk.
1975		J. Carr Bad (1995) 45: The guys are really gonna rip you off, that’s no shit.
1990		(con. c.1970) G. Hasford Phantom Blooper 21: This is no shit, pilgrim. The true story of the War for Southern Independence.
2017		D. Winslow The Force [ebook] ‘I’m not small,’ she says [...] ‘I’m concentrated.’ Which is no-shit true.
2023		(con. 1962) J. Ellroy Enchanters 418: I was bughouse / no-shit scared.

In derivatives

no-shitter (n.)

a true statement.

1992

R. Marcinko Rogue Warrior (1993) 209: Tom, I’m about to give you a no-shitter.