What popularized "a roll in the hay" in the 1940s?

While I was looking for an expression for casual sex I came across the evocative expression "a roll in the hay."

The saying is from 1942 according to Etymonline:

Meaning "act of sexual intercourse" is attested from 1942 (roll in the hay).

Where does this expression come from? Was it from a movie or a romantic book of the forties?

What are other similar expressions?

The popularity of the phrase 'roll in the hay' in the 1940s, such as it was, was perhaps attributable to the action in a 1941 book made into an Academy Award-winning movie: This Above All, by Eric Knight.

At first, I was unconvinced that the phrase enjoyed any special popularity in the '40s. Google Ngrams shows a different picture.

The occurrences linked below the Ngram also do not bear out the claim: 1800-1942 gives only two references, a 1934 book of that title described as "English wit and humour", and an 1810 song book; 1943-1990 gives only five references, including another book titled A Roll in the Hay, this one from 1979 and described as "Paraphilias".

As usual, however, Google Ngrams proved deceptive. COHA for the phrase 'in the hay' showed two usage bumps in that corpus, in the 1920s and 1940s:

Examining the context of the uses in the 1920s and 1940s, it can be seen that 'in the hay', while instanced twice with a prefaced roll in 1941, is used with other senses and constructions in the 1920s. The two exact instances of the phrase in 1941 both refer to sex:

She looked at me and I saw how beautiful her eyes could be when they weren't being hard and I met the look, inquisitively, and she turned away to stare out at the lights far below. " Hell, this isn't a roll in the hay for me, there's plenty of that around. This is, well, maybe the best way of saying good-bye... "

(From What Makes Sammy Run?, Budd Schulberg, 1941.)

"Not a place for picnicking, or a roll in the hay," he said.

(From Mildred Pierce, James Cain, 1941.)

1920s:

1940s:

Note that none of the 12 instances from the 1930s used the exact phrase 'roll in the hay'; one of those instances was, however, synonymous with 'roll in the hay': 'sported together in the hay'.

Of the 11 instances in the 1950s, five are in the sense of 'sex in the hay':

... You and me would be terrific in the hay, baby.

(From Return to Paradise, James A. Michener, 1951.)

If he should end up in the hay with Angela tonight ...

(From Rally Round the Flag, Boys!, Max Schulman, 1957.)

They gave me a gold star or a roll in the hay with the principal's wife or something.

(From Plowshare in Heaven, Jesse Stuart, 1958.)

... if you stayed on you'd wind up in the hay with Mama.

(From Hawaii, James A. Michener, 1958.)

... I did not wind up in the hay with Mama.

(op. cit.)

From these data, which show three instances of use of the phrase 'in the hay' in This Above All in 1941, and acknowledging the popular appeal of This Above All, both the book and the 1942 movie, with its plot-line that includes a central character, Clive, who is disgusted enough by the war to go AWOL,

having a crisis of conscience over what the war is being fought for and disgusted at the incompetence of the ruling elite, Clive decides not to return to the Army and to go absent without leave,

(Wikipedia, "This Above All")

and so as a sop for the US people's ambivalence about US involvement in the war at the time,

a large majority of the American public continued to oppose any direct military intervention into the conflict well into 1941,

(Wikipedia, "World War II")

the reason for the popularity of the phrase in the 1940s may be explained. The exact phrase 'roll in the hay' did not, so far as I know, occur in the book or the movie; the action of the book and the movie both, though, perhaps contributed to the adoption of the phrase (and particularly Michener's repeated use of it in the 1950s) with the particular sense of, if not casual, at least carefree sex.

Other similar expressions are numerous. I'll list a few I've heard or seen in use, but an exhaustive listing is quite impossible. For more, beyond my limited experience, see for example the list at The Online Slang Dictionary.

Phrases:

• a bit of how's your father,
• beat cheeks,
• bump uglies,
• dip [your] wick,
• do the horizontal bop,
• do the nasty,
• get down and dirty,
• get laid,
• get lucky,
• go at it,
• make woopie,
• slap and tickle.

Etc.

By way of offering a minor supplement to JEL's fine answer, I note that, in an earlier day, "rolling in the hay" was an innocent activity that children might indulge in at a farm. Thus, for example, in "Sunshine for the Children Urged by Queen Helene," in the Washington [D.C.] Times (November 3, 1909), Queen Helene of Italy recalls the happy hours her children enjoyed in nature:

My two little girls an my small boy never know what it is to be restrained in the enjoyment of nature. They run barefoot at the seashore: the ocean, with proper restrictions of course, is their tub. You should see them roll in the hay at our farm, or watch them in the green, or in the snow, at the Quirinal gardens.

From a letter to "Uncle Jeff" from Marjorie Gordon ("13 years 3 months") of Wolseley Park, New South Wales, in "The Children: For Young Australians Who Read 'The Banner'," in the Albury Banner and Wodonga Express (December 9, 1910):

It will soon be haymaking time, and then it is lovely to roll in the hay, climb the haystacks, and smell the perfume of the hay in the early morning. A town life is made disagreeable by so much dust and noise; and you cannot go for irdes like you can in the country.

From "'Patricia Pat's' Weekly Letter" in the Queensland Times (June 3, 1930):

It might be remembered that when Mrs. Moore lived with her husband at their home at Wailawa, Jondaryan, she took her turn at milking when labour was scarce, and during Mr. Moore's campaign in the North, his practical wife kept an eye on the cheese factory which was on their property. She was an adept at preparing returns and milk sheets, and such things which are all important in the satisfactory running of a cheese factory. Many a time, too, she has helped bring in the hay, and I suppose has enjoyed the sensation which youngsters love when they have a good roll in the hay, and come home all scratched and red. Au revoir. "PATRICIA PAT."

From Ladbroke Black, "Nell Gwynn" in Serial Form: Piquant Story of Sweet Nell as Written for the Films," in The [Adelaide, South Australia] Mail — (February 23, 1935):

"Oh, a buxom lass is a cuddlesome thing

For a soldier, sailor, beggar man—king.

For the smile of a maid for a love-lorn lad,

Be he beggarman or king, makes his heart glad.

With a bullywhack, a gillywhack, a gillywhack-a-lo,

We'll roll in the hay when the sun gets low.

Oh! Your ladies grand in their powder and paint,

They may be what they seem or be what they aint.

But there'll still be them wouldn't have them as a gift

Who would give them all the go-by just for Nelly in her shift.

With a bullywhack, a gillywhack, a gillywhack-a-lo,

There's a kiss in the dark for a lad I know.

And from Margaret Barnes, Wisdom's Gate, in Australian Women's Weekly Novel (September 9, 1939), there is this unusual conversation:

Belle's voice arrested her. "Don't go, old girl. And don't be offended. I can't help seeing that you're in a jam. I assume, from experience, that Albert's been outrageous, but ——"

"Oh, please—!" said Cicily, with a little helpless gesture.

"Of course, I don't know exactly what you want," Belle began, rather hesitantly.

Cicily said nothing, made no movement, to enlighten her.

Belle drew her own conclusions from the miserable pause. "If you go on like this, you'll lose him," she said warningly. And then, "I should know."

Cicily blushed to the roots of her hair.

"It was lucky for me that you took him off my hands," Belle hastened to assure her. "I was stupid not to realise it. I'd been nagging him for years. He simply hated it, and it was all cold mutton. But I think this is different. He was your big moment and you've never got over him. But, Cicily, you can't change him. He will always remain sublimely the same. I'm sure he's in love with you—he respects and admires you—but of course he will always roll in the hay——"

"Belle—!" cried Cicily, shocked to sharp protest.

"Can't you overlook it?" Belle calmly inquired.

Cicily felt ready to burst into tears.

"If you want to keep him, you'll have to put up with it."

There can hardly be any doubt that Queen Helene, Marjorie Gordon, and "Patricia Pat," writing in 1909, 1910, and 1930, have no innuendo——or indeed anything but literal rolling in hay—in mind when they use the term "roll in the hay." The situation is less clear in the 1935 "Nell Gwynn and 1939 Wisdom's Gate instances, but it may be that the figurative sense of "roll in the hay" in these cases falls short of any suggestion of sexual intimacy.

In that case, it follows that the 1941 instances that JEL identifies may be the first to imply a sexual dalliance.