The colloquial phrase couldn’t stop a pig (in a gate, or in an entry, etc.), and variants, are used of a bow-legged person.

Incidentally, the converse metaphor occurred in a letter to the Editor, published in The Morning Chronicle (London, England) of Tuesday 1^st May 1832:

Peter, his son, was […] a collar maker; and used to work from six in the morning until eight at night, sowing collars between his knees; so that you will find he is, at this time, knock-kneed and excellently well made to stop a pig.

—Cf. the phrase can eat an apple through a picket fence, which similarly derides a person’s physical characteristic.

The earliest occurrences of the phrase couldn’t stop a pig (in a gate, or in an entry, etc.), and variants, that I have found are as follows, in chronological order:

1-: From Oxberry’s Dramatic Biography, and Histrionic Anecdotes (London: George Virtue, 1826), by the English actor William Oxberry (1784-1824) and Catherine Elizabeth Oxberry (née Hewitt – c.1790-?)—the following is about the English stage actor Junius Brutus Booth (1796-1852):

On one evening, when we submitted to the operation of squeezing up a passage of steps about eighteen inches wide, we saw the aforesaid Mr. George enact King Richard the Third, and Booth was the Buckingham of the night, and a very promising performance his was: one circumstance, however, occurred, which certainly did not say much for the elegance of the auditors. Mr. Booth, it must be premised, and tenderly be it spoken, was in early life rather strangely formed in the leg, or, as it may be more delicately stated, defective in his understanding. A curve has been, for ages, allowed to be the line of beauty, but this line will not do to apply to the legs: well, then, whilst Mr. Booth was perpetrating the Duke of Buckingham, a gentleman in front volunteered the following shrewd remark:—“Ah, ah! you’re a pretty fellow to stop a pig!” we need not add, that the laughter of the auditors confirmed the truth, if not the kindness of the observation.

2-: From Theatricals, published in The Age (London, England) of Sunday 13^th December 1840:

The Thespians, as they call themselves, par excellence, attempted the performance of Othello, on Monday last, at the Richmond Theatre. The house was very badly attended, and those who were absent certainly may console themselves with the knowledge that Shakspere himself would never have recognized his bantling in such hands. To be sure, those who are fond of fun missed a hearty laugh, which was only enjoyed by the select few present. The person who enacted Brabantio was a tall knock-kneed youth, without any reverend disguise, if we except a smudge of dirt on each side of his nose, precisely as if his proboscis had been tweaked by a “chummy.” Add to this the fact, that either from stupidity or “funk,” he could not remember a word of his part, our readers may conceive the object who claimed the paternity of the gentle Desdemona. The Doge of Venice was in the same oblivious state, but fortunately had a copy of his author on the table before him. Roderigo was entrusted to a youth, whose legs were wide asunder, like the poles, or as a gallery wit tritely observed, a rum customer “to stop a pig.”

3-: From Recollections of Oxford, published in Once a Week. An Illustrated Miscellany of Literature, Art, Science, & Popular Information (London: Bradbury & Evans) of Saturday 21^st January 1860:

Billy Bouquet, or, as he was called by undergraduates who were shy of French, Sweet William, somewhat resembled in personal symmetry Mr. Robson’s “Boots at the Swan.” His head, which gave one the painful idea of having been sadly overgrazed by his rats, was screened from the inclemencies of our fickle climate, and made symbolical at the same time of his avocations and attachments, by a memorial cap from the epidermis of a deceased bull-dog, of whom he was wont to remark, in all seriousness, that “he’d always know’d that his dog Beerhouse” (archæologists assure me that his original name was Cerberus) “was a sight too good for this world.” His neckerchief had once been scarlet—a præ-Raphaelite, vivid scarlet—but time and perspiration had done their silent work, and it was now a peaceful brick colour. His coat and vest of velveteen (the bronze buttons chastely relieved with foxes’ heads in the last stage of inflammation) were noticeable for their vast infinity of pockets, one of which, inside the coat, I verily believe would have held a calf. The rest of his person was clad in kingly cord; and of his legs I have only to say, that he was the very last person whom you would have selected to stop a pig in a gate, for the obvious reason that the animal in question would most undoubtedly have run between them.

4-: From The Leeds Intelligencer (Leeds, Yorkshire, England) of Saturday 28^th November 1863:

The Bairnsla Foaks’ Annual an Pogmoor Olmenack fur 1864. Be Tom Treddlehoyle, Esq. (Leeds: A. Mann).—This favourite illustrated comic annual, written in the vernacular of “black Barnsley,” is marked by the peculiar vein of humour which has secured for the emanations of the pen of “Tom Treddlehoyle” a ready welcome from, we may say without exaggeration, many thousands of his fellow-countrymen. The present yearly part is of the same size and is got up in the same style as those of past years, and it is illustrated with about the same number of comic engravings. The following extracts may serve as samples of some of “Tom’s” shorter observations:—
Spendin hiz Strength fur Nowt.—A bow-legged fellah tryin ta stop a pig e a entry.

5-: From the following by ‘Rose Rampant’, quoted by the English Anglican priest, author and horticulturalist Samuel Reynolds Hole (1819-1904), in The Gardener: A Magazine of Horticulture and Floriculture (Edinburgh and London: William Blackwood and Sons) of May 1869:

THE ROSE.
CHAPTER X.—GARDEN ROSES.

Soon after the publication of my last chapter, I received from a furio-comic amateur the following epistle:—
Sir,—I wish to be informed what the Two in Whist you mean by leaving me on the 1^st of April, ult., in a ridiculous costume and a crowded anteroom, quietly proposing to keep me there for a month. My legs, sir, cannot be included among “varieties suitable for exhibition.” They have, on the contrary, been described too truly by a sarcastic street-boy as “bad uns to stop a pig in a gate,” and you might at least have clothed them in the black velvet trousers recently and reasonably introduced.

6-: From Down the Mississippi, by ‘C. L. A.’, published in The Birmingham Daily Gazette (Birmingham, Warwickshire, England) of Wednesday 13^th July 1870:

General Butler, who was, I have heard, originally a lawyer, and did a large amount of dirty pettifogging business in that line in a small town not far from Boston, in the State of Massachussetts [sic], on the breaking out of the war got a commission as General in the army of the United States, and was first sent as military governor to Baltimore, in which city he distinguished himself by many acts of brutality and cruelty. He was afterwards sent to take possession of New Orleans, and, while there, quite justified the character he had earned in Baltimore—in appearance (for I have had the honour (?) of an interview with him). This modern Nero is anything but prepossessing; he is short, square built, with legs best described as not calculated to stop a pig in an entry; he has a broken nose, and only one good eye (the other has a cut or scar that gives a most repulsive expression to his face), and one of the many epithets applied to him, that of “Butcher,” best described his tout ensemble.