Green’s Dictionary of Slang

nah! excl.

(US) no!

1927		AS II:11 475: List of the variants of ‘no’ [...] nope, nit, nitsky, nix, nixy, nah, naw, aber nit, not on your life, nay, nay, Pauline.
1961		V.S. Naipaul House For Mr Biswas 363: Nah. But suppose he ask her to go to town and buy something.
1985		S. Berkoff West in Decadence and Other Plays (1985) 102: steve: Seein’ Sylv tonight? mike: Nah.
1995		Eble Campus Sl. Dec. 5: Nah, G – emphatic no.
1999		K. Sampson Powder 86: Nah. They’re just enjoying it all. That’s all.
2000		Indep. Rev. 1 Mar. 3: Nah, don’t be ridiculous.