Some porcine-related phrases make clear sense, such as “pigging out” or “happy as a pig in mud.” Pigs also have a reputation for excessive perspiration, given the popularity of the phrase “I’m sweating like a pig!” However, this seems to be just bad press. According to 4-H Agent Matthew Newman, while swine do have some sweat glands, as all mammals do, they have relatively few for their size. As such, they have to roll in the mud or do another similar activity to cool their bodies on a hot day. So, where did “sweating like a pig” come from? The idiom actually has to do not with the animal but with the process of iron smelting. More specifically, it comes from the term “pig iron.”
“Pig iron,” going back to 1665, refers to the “crude iron that is the direct product of the blast furnace.” It’s known as “pig iron” because of the way iron used to be cast. Hot iron was poured into sand molds in a way that visually resembled tiny piglets suckling at their mother’s teat. When the ingots cooled and were broken off, they were sometimes referred to as “pigs,” hence the term “pig iron.”
As for the “sweating” element of the phrase, it has to do with the cooling process. Dr. Joe Schwarcz from the McGill University Office of Science and Society explains that as iron cools, the surrounding air begins to hit its dew point. In turn, this causes moisture to form on the iron, which looks like little droplets of sweat beading down the ingots (the pigs); this is where we get “sweat like a pig.”
Advanced smelting techniques have largely rendered pig iron a thing of the past, though the phrase maintains its impressive staying power. Today it can be a reminder of older smelting techniques, but it has nothing to do with Porky or Babe.
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