That is a large sum; that is not insignificant •Chopped liver is very commonly used analogously; cornflakes and zucchini are also found, but rarely: And what they pay me in addition ain't hay(1943+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with that ain't hay
That's a great deal, especially of money; also, that's important. For example, He's making ten thousand a month, and that ain't hay. Originally used to describe a sum of money that is large, this phrase was later extended to other circumstances, as in She married a titled lord, and that ain't hay.
[ ; first half of 1900s