Green’s Dictionary of Slang

(usu. US milit.) no matter, ‘I’m easy’, ‘that’s cool’.

1952		J. Niles [bk title] Mox Nix: Anecdotes about the Life of GI’s in Europe.
1963		‘Letter from an AA’ in US Army Aviation Digest Jan. 17/1: This may seem a bit odd, but is best explained by the frequently heard term, ‘Mox Nix’.
1973		D. Ponicsan Cinderella Liberty 63: The sitzers pass around a joint. Mox nix to Baggs. He hits on it.
1987		R. Merkin Zombie Jamboree 83: Mox nix, Whitebread. I’d loan you a razor blade, but all I have is those cartridge jobs.
1998		Spillane & Collins (eds) Private Eyes 292: Mox nix. The game's over . You lose.
2006		T. Haag [bk title] Mox Nix: A Tale of Corporate Corruption, Chemical Toxicity.
2013		Dunn & Aragones Zounds! 114: Mox nix gained popularity among American soldiers during the period of Allied occupation of Germany after World War II. Its use is moribund these days.