Coined after grass widow, the expression grass widower designates a married man living apart from his wife, either temporarily or permanently—especially a man whose wife is away often or for a prolonged period.

This expression occurs, for example, in the review of A Love Undone, a novel by Cindy Woodsmall—review published in the Daily American (Somerset, Pennsylvania, USA) of Wednesday 8^th October 2014:

Andy is cautious about his deepening friendship with Jolene, but he believes she knows the truth about him—that he is a grass widower. As a man whose wife has abandoned him six years past, he is unable to divorce or remarry according to the Amish ways.

The earliest occurrences of the expression grass widower that I have found are as follows, in chronological order:

1-: From Memoirs of Mrs. Q——, published in Rambler’s Magazine: or, Man of Fashion’s Companion (London, England) of Monday 1^st July 1822:

His lordship soon after married a Piccadilly belle, with a 50,000l. fortune. She now lives in France with an Irish fencing master, and his lordship is upon the town as a grass widower.

2-: From The Chester Chronicle, and Cheshire and North Wales Advertiser (Chester, Cheshire, England) of Friday 27^th July 1832:

The Markets—Unwholesome Meat.—We are informed there is every week meat exposed for sale in our markets which is utterly unfit for human food—absolute carrion, both from disease and putrefaction […]. We caution the public against buying pieces of “fine corned beef” as it is called, from persons who offer it for sale. For the most part, it consists of tainted meat for which no sale can be obtained in the ordinary way, and sometimes a slice of the buttock of a dead horse—if a fat one, so much the better. A friend of ours, at that time “a grass widower,” bought what he thought a superb round of corned beef, from a butcher, that his friends might “cut and come again” during the race week; but when it was boiled, *** we forbear the description—it might produce the cholera this warm weather!

3-: From the transcript of the testimony given by one Francis Brunker during a trial at the Court of King’s Bench, published in Saunders’s News-Letter, and Daily Advertiser (Dublin, County Dublin, Ireland) of Saturday 18^th June 1836:

I am a shoemaker; I am a married man; I used to frequent Hayes’s house as a place of enjoyment after my shop has been shut; […] my wife was never at the place; as long as I was a grass widower I have frequented the house; I have played cards at Hayes.

4-: From A Maltese Adventure. A General’s Rivalry, in Stories of Torres Vedras (London: Richard Bentley, 1839), by the British author John Gideon Millingen (1782-1862):

I had heard from the captain of the vessel, somewhat more intelligent than the generality of transport skippers, and who had made many voyages to Malta, that Mr. ——, the Russian lady’s husband, had bestowed his affection on a lovely Sicilian girl. This intelligence I determined to turn to some account.
Upon our arrival in the beautiful harbour of La Valetta, the first thing I did was to possess myself of the key of the lady’s trunk, and throw it overboard. This I knew would delay her landing for a considerable time […]. I forthwith beckoned one of the boatmen upon deck, and entrusted him with the following epistle to Mr. ——
“Sir,
“Although I have not the honour of being personally known to you, yet as I hope that I may put in some claim to those blessings that may be bestowed upon peace-makers, I hasten to inform you that your amiable lady is on board the ——. As sudden meetings are sometimes too overpowering after a long absence, I have thought it prudent to give you this piece of intelligence for your government. In the meantime, I shall endeavour to detain your lady on board by every possible means.
“A Friend.”
[…]
It was evening before we could land; I had the enviable task of accompanying her to her husband, who played the part of a surprised grass-widower extremely well, expressing his delight at the unexpected happiness.

5-: From a correspondence from Baltimore, dated Saturday 12^th February 1842, titled The Green-eyed Monster, &c., by ‘Roderick’, published in The New York Herald (New York City, New York, USA) of Sunday 13^th February 1842—I have not corrected the misprints:

Not many days since, a disconsolate old man, bearing the evidence of some fifty winters upon his brow, was seen at one of our principal hotels, soon after he was heard of in the vicinity of Frederick, then again in Baltimore, then at Anapolis, and lastly at Washington. Upon instituting an inquiry as to the cause of his eratic course, or wild-goose chace, it was ascertained that he was, not like Celebs in search of a maiden of whom to make a wife, but like a grass widower, run mad in pursuit of his once adorable and affectionate bosom partner.