The phrase to sweat like a pig means to sweat profusely. The earliest instance that I have found is from The Morning Post (London) of 10^th November 1824; during a boxing match “between Ned Turner and Peace Inglis for one hundred pounds a-side”, one Shelton, who waited on Turner, said of Inglis:

“He sweats like a pig.”

A variant is found in the Berkshire Chronicle (Reading, Berkshire) of 24^th October 1829; an article recounts that, in the middle of a night, the English pamphleteer and journalist William Cobbett (1763-1835) asked his friend, Riley, to keep him company during a thunderstorm; Riley remained in Cobbett’s bedroom

until the last gleam of lightning, and the very last faint groan of the dying tempest, permitted him to depart. Upon turning his eyes towards Cobbett, as he (Riley) was quitting the room, what should he see but his huge head just emerging from under the blankets and the superincumbent pillow, his two red chubby cheeks, to use a simile a la Cobbette [sic], dripping with sweat like a pig!!!

ORIGIN

The phrase to sweat like a pig is difficult to explain. In Food and Culture: A Reader (2013), Carole Counihan and Penny Van Esterik explain that it lacks an anatomical basis, as pigs can’t sweat—they have no functional sweat glands. In order to keep cool, the pig wets itself down with moisture derived from external sources:

Here, then, is the explanation for the pig’s love of wallowing in mud. By wallowing, it dissipates heat both by evaporation from its skin and by conduction through the cool ground. Experiments show that the cooling effect of mud is superior to that of water. Pigs whose flanks are thoroughly smeared with mud continue to show peak heat-dissipating evaporation for more than twice as long as pigs whose flanks are merely soaked with water, and here also is the explanation for some of the pig’s dirty habits. As temperatures rise above thirty degrees Celsius (eighty-six degrees Fahrenheit), a pig deprived of clean mudholes will become desperate and begin to wallow in its feces and urine in order to avoid heat stroke.

However, there are two types of sweat glands:
– the eccrine glands: their water-based secretion regulates body temperature;
– the apocrine glands: the substance secreted is thicker than eccrine sweat and provides nutrients for odour-causing bacteria on the skin.

While pigs have no eccrine glands, they have apocrine glands, and, according to one theory, to sweat like a pig refers to the smell produced by the bacteria growing on the pig’s skin.

But, since all domestic mammals have apocrine glands, why does the phrase specifically refer to the pig?

In Is That a Fact? Frauds, Quacks, and the Real Science of Everyday Life (2014), Joe Schwarcz is certain about the origin of the simile:

That expression is actually derived from the iron-smelting process in which hot iron poured on sand cools and solidifies. The resulting pieces are said to resemble a sow and piglets*. Hence, “pig iron.” As the pigs cool, the surrounding air reaches its dew point, and beads of moisture form on the surface of the pigs. “Sweating like a pig” indicates that the pig has cooled enough to be safely handled.

(* Since the late 16^th century, the word pig has been used to designate an oblong mass of metal as formed by molten metal run from a furnace and allowed to solidify. The word sow is found in this sense as early as the late 15^th century. The original differentiation of sow and pig, if there was any, was probably in the size, the smaller masses being called pigs.)

But Joe Schwarcz should not be so categorical, because it is doubtful that a technical expression originating in limited circles would have given rise to a popular phrase. Additionally, to sweat like a pig appeared in the 19^th century only, more than two hundred years after pig had come to denote an oblong mass of solidified metal. Finally, in the first instances of the phrase to sweat like a pig, there is no connexion whatsoever between the word pig and metal pieces.

The possibility that to sweat like a pig is borrowed from, or at least related to, an Italian simile seems to appear in a letter from Naples, dated 15^th April 1826, in which the English painter Thomas Uwins (1782-1857) described an Italian monk to one of his brothers, Zechariah:

He is a good, fat, haughty-looking fellow, and the exertion of going on his knees, laden as he was with robes, made him ‘sudare com’ un porco,’ that is ‘sweat like a pig.’ This is an expression very common in an Italian’s mouth, even with ladies, but it does not sound very well in English.

But no evidence supports this connexion: apart from this instance, the early attestations of the English phrase have no relation with Italian or Italy.

The more probable explanation is simply that the pig has come to symbolise the unpleasant fact of sweating profusely in the same way as it often represents greed, dirt, etc., in many derogatory idioms — cf. for example to eat like a pig, to make a pig of oneself and happy as a pig in muck.