Argthtjtr

I have trouble finding anything about this. Google offers several quotes at the top of the results:

• Rowena Cory Daniells, "The King's Bastard", 2016

'You fancy her!'
Lence nodded, 'And what's more, I've tasted her sweet fruit.'
'No, you never!'
'Autumn cusp, in the hay after the Harvest Feast.'
[...]
'She deserves Better than a fling in the hay, Lence.'
'Well, that's all I can offer, remember?' Lence snapped. 'I'm to be married to the Merofynian kingsdaughter. So, go ahead, woo her, marry her if she'll have you. But one day she will be my mistress.'

• Elizabeth August, "Cinderella Story 3, 2014

"Besides, I like my independence. All I want is a little fling in the hay once in a while to satisfy my more primitive cravings, then I'm happy to be on my own again."

ngram finds nothing though, not even these google-books books.

It's pretty obvious that the saying relates to the "affair" definition of fling. So the preliminary question must be, which was first, is fling in this sense just an ellipsis?

The more current idiom seems to be a roll in the hay, agreeing roughly with the trivial meaning of fling "to throw". There, it's noted that hay can be a synonym for bed, and that roll in the hay can mean inconspicious children's play, frolicking. One has to say had meant, actually, because the the daily live has changed and thus turned the euphemism overt, occluding the once obvious meaning.

Yet, I think I can read an inuendo in 1838

Ingraham, J. H. (Joseph Holt) "Burton; or The Sieges Volume 2]()", 1838

If it had been the spy-monk in thy case, he would not have let a maiden tuck him up in the hay, and leave him there to go to her lone pillow.

I'm trying to read hay as hay field, at least in some older instances of "in the hay", because I first associated German Springinsfeld (lit. jump into the field; It's an appellation, not one of the usual long German compound words, it appears like a speaking name, like Hans Guckindieluft "Johnny Look-at-Air"), although I'm not sure what a "hay field" should be, if long grass becomes hay only after the harvest. Anyhow, there the answer currently says it's military jargon, which needn't be original. As far as I know, "fling in the hay" does not usually describe a person, but it comes rather close e.g. in "All I want is a little fling in the hay".

I'm hoping a quick fling in the hay can elucidate the question. Give me your best shot at the origin of the phrase. Anything far flung is welcome, too, if it can be possibly backed-up with sources.

"Fling in the hay" is a slightly mixed expression combining a "fling", meaning a brief sexual dalliance with no commitment, and "roll in the hay" meaning a casual sexual encounter (which may or may not be literally in the hay, depending on environment, but is probably somewhere other than a comfortable bed).

The modified expression probably extends "roll in the hay" to a few encounters, but not to the level of any kind of real relationship.

The instances of 'fling in the hay' I've been able to uncover in early (before 1995) popular news venues suggest that the phrase may have emerged as a synonym of 'roll in the hay' sometime before the 1980s.

The first appearance I found, in a hostile and contemptuous 1981 review (paywalled) of a stage play, A Very Modest Orgy, greatly embellished the phrase, and seemed to be chiefly an attempt to dress up 'roll in the hay' with more contemporary language:

Phoebe...may well want to test the unlikely sensual possibilities of a whipped-cream fling in the hay loft with another couple willing to swap sexual stamps.

Edmonton Journal (Edmonton, Alberta, Canada) 13 Oct 1981, p. 31.

In the review, 'fling' means "a short period of unrestrained pursuit of one's wishes or desires" (Dictionary.com).

The second appearance I found was a 1983 teaser (paywalled) for an advice column:

So, if his wife wants to have a little fling in the hay, what the hey?

News-Press (Fort Myers, Florida) 05 Nov 1983, p. 41.

This 'fling' means what it did in the earlier use. All told, I don't see a material difference between the meaning of this "fling in the hay" and the meaning of the more common phrase 'roll in the hay'.

The third appearance I found was, like the first, embedded in a hostile review; the subject of the 1984 review (paywalled) was a mini-series TV adaptation of Hemingway's The Sun Also Rises.

It concerns Jake Barnes, ...who has been emasculated by a German grenade — a matter of great concern to Lady Brett Ashley, who loves him dearly but can't do without her flings in the hay.

It could be argued with some justice that "rolls" would not work as well as "flings" in that sentence. However, the meaning is the same: 'fling' is used in the sense cited above, and 'the hay' is used in a sense recorded by OED (paywalled):

the hay: colloq. phr. for 'bed'; esp. in phrases to roll in the hay.

The fourth appearance of 'fling in the hay' I found in a 1987 promotional blurb (paywalled) for the 1964 James Bond movie Goldfinger:

The movie is remembered...for Bond's...fast fling in the hay with Pussy Galore....

All told, then, the sense of 'fling in the hay' I'm understanding from the uses so far is "a short period of unrestrained pursuit of one's wishes or desires" "in bed". The phrase with 'fling', like the phrase with 'roll', refers to "a brief sexual liaison". If there is any difference between the phrase with 'fling' and the phrase with 'roll', it is that 'fling' more readily admits interpretation as being a longer period than 'roll'.

The 1980s seem to have been the heyday for the 'in the hay' phrase with 'fling'. After 1987, the next appearance I found was in 1995, eight years later. After that appearance, 'fling in the hay' in the print press (at least so far as the Newspapers.com corpus is concerned) disappears for nine years, reappearing in 2004. Both the 1995 and the 2004 appearances were uses in the same sense as 'roll in the hay'.