1
general::
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. [Colloquial; first half of 1900s]
American Heritage Idioms
2
general::
Phrase(s): That ain’t hay.
Inf. That is not a small amount of money. (The highly informal word ain’t is built into the expression.) • I paid forty dollars for it, and that ain’t hay! • Bob lost his wallet with $200 in it—and that ain’t hay.
McGrawhill's American Idioms And Phrasal Verbs