The phrase all behind, like a cow’s tail and its variants have two meanings: left behind and late in accomplishing a task. For instance, in her cooking-recipe column, in the Great Falls Tribune (Great Falls, Montana, USA) of Wednesday 16th November 1994, Jackie Rice introduced a recipe in this manner:
Faith Hagen sent a tomato soup recipe in response to a request made last month. “Sorry I didn’t get this into the paper sooner, but I’m always behind like a cow’s tail,” she wrote. We’re even farther behind in getting it into print. Thanks, Faith.
—Synonym: all behind like Barney’s bull.
In Dictionary of English Phrases (Penguin Books, 2008), Robert Allen writes, about all behind, like a cow’s tail, that “there is no evidence before the mid-20th century”, but I have discovered that the phrase appeared in print in the mid-19th century in the USA, Australia and Britain.
The earliest instance that I have found, about a horse race, is from The Daily Picayune (New Orleans, Louisiana, USA) of Friday 29th May 1857:
The race was mile heats, three in five, trotting: and the entries were Jack Cassidy, Queen and Snow. The first named is a stranger among us […]. The other two are old acquaintance […].
In this close contest between two white-haired neighbors the stranger, Cassidy, was lost sight of. He came in third on the first heat, but was all over behind, like a cow’s tail, in the second.
On Monday 4th February 1861, The Sydney Morning Herald (Sydney, New South Wales, Australia) published a letter from “a working man” addressed to the working men of New South Wales, containing:
There’s a prejudice among working men […] that immigration does harm. Now, once before I showed you what a blunder this was, and how as a matter of fact the labourer had always been best off when immigration went on fastest. […] I see the folks in Victoria are beginning to open their eyes on this point, and mean to restore immigration as fast as they can. But we’re all behind, like a cow’s tail.
On Saturday 27th April 1861, The Carmarthen Weekly Reporter (Carmarthen, Carmarthenshire, Wales) published a letter in which a person signing themself Sentry wrote that the phrase was Irish-English slang (which is doubtful, considering the early instances):
According to a popular author, the “march of intellect,” with other social qualities, has made monster strides the last few years, although it is still apparent that there are yet living individuals who, from an innate crudeness, seem to eschew keeping pace with the progress of the nation, and, as Pat in Hibernian slang said, they are all “behind, like a cow’s tail.”
Finally, the following is from The Cornishman: a newspaper for “one and all.” (Penzance, Cornwall, England) of Thursday 10th February 1881:
WHAT THE CORNISH MINER WANTS.
Sir John St. Aubyn you da know is a very good member, an’ all the St. Aubyn’s hev’ advocated from their earliest days the miners’ cause, an’ he da say some years ago at a ploughing match that Jan and Will, two Cornish miners, laid a wager which of ’om was the best ploughman. The umpire was appointed, each of ’om had his chance an’ turn at ploughing, an’ the umpire he says—‘Jan, thy ploughing is the worst I ever seed in my life, an’ as for Will’s, ’tes no ploughing at all.’ Well, now, Pudden’ Will, unless we da git the dynamite cheap the same will be said of we—that we’re no miners at all, nor doant understand mining, and we shall be nowhere, for everybody will be gwain afore us, for you do know as well as me that the dynamite bill is a nation heavy one, an’ a very soon break our backs when the mine es rather low in money, an’ I da hope we shall git un cheap, fur a ’es a grand thing, an’ ef we doant hev’ un we shall be all behind, like a cow’s tail. But if they’ll only give us good steel, good fuse, good dynamite, an’ a sensible capt’n that give we a fear price, then, Pudden Will, with the motter ‘One an’ All,’ an’ a long pull an’ a strong pull altogether, we’ll take our proper stand, an’ beat us who can. That’s what I say.”