Green’s Dictionary of Slang

(US) a phr. used to mean that something is a large and/or significant amount.

1851		‘How Sally Hooter Got Snake-Bit’ in T.A. Burke Polly Peablossom’s Wedding 68: If you didn’t think all the peas in my corn field was er spillin in the floor, thar ain’t no ’simmons!
1942		N. Algren Never Come Morning (1988) 98: It ain’t hay in here.
1944		J.M. Inks diary Eight Bailed Out (1954) 8 Aug. 51: Fifty thousand bucks ain’t hay even in Texas.
1947		J. Pagano Condemned 64: I got three hundred bucks muster-out pay, the generous bastards, but my job, seventy-five bucks a week and that ain’t peanuts.
1957		L. Hughes Simply Heavenly II ix: I make five or ten dollars, sometimes more a day. You men what ain’t working know that that ain’t hay.
1975		T. Berger Sneaky People (1980) 91: He had a hundred and eleven dollars in the bank, and that ain’t hay.
1978		Phi Delta Kappan LX 378: My rank is 278 out of 900 and that ain’t peanuts.
1983		J.C. Wood John Maynard Keynes 278: Fifty billion dollars for every excess point of unemployment — that ain’t peanuts.
1999		Quest LI 99: But that’s the difference between $1.1 billion and $1.5 billion, and that ain’t peanuts.
2003		B. Weltman Complete Idiot’s Guide to Starting a Home-Based Business 30: 5 percent of a million is 50000, and that ain’t hay.