Why Do We Say ‘Sweat Like a Pig’?

In moments of great duress, a person may be observed to be “sweating like a pig.” It’s an inelegant phrase, and one that doesn’t hold up to much scrutiny. Pigs, after all, don’t thermoregulate like humans and therefore don’t sweat in any conventional sense, much less sweat so much as to have the analogy make sense.

So why do we say people experiencing anxiety or nervousness are perspiring like hogs? Shouldn’t we instead say they’re sweating like a bomb squad? Or sweating like a marathon runner?

It’s because the phrase is actually referring to a different kind of pig. In iron smelting, hot, liquid iron was poured over sand to cool. The molds used for this process looked to some like a suckling pig silhouette, which gave the material the nickname “pig iron.” Once cooled, it became useful for the production of things like frying pans, cookware, fencing, and fireplace components.

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The sweating comes in when the iron begins to cool. It meets the dew point of the surrounding air and moisture collects on the surface. These “pigs” are perspiring in a metaphorical kind of way, signaling the iron is getting comfortable enough to handle.

The phrase is therefore not referring to pigs at all but the colloquial term in the ironmaking process. It would be more accurate to say "sweating like pig iron."

Do pigs actually sweat?

Pigs may not sweat much—at least not visibly enough to make this cliché stick—but they do sweat. The problem is, it’s usually not enough to reliably cool them down. Pigs have fewer sweat glands than a mammal of their size needs to maintain a consistent body temperature, so they tend to gravitate toward external processes like rolling in mud to offset the heat.

The moisture in mud or a body of water acts in a manner similar to sweat. As it evaporates on a pig’s skin, it has a cooling effect. Pigs may also seek shade or even pant to control their body temperature. (Another porcine phrase, “happier than a pig in slop,” makes much more sense as a result of this heat reduction strategy.)

In hotter weather, it might also be a misnomer to chide a pig for their dining habits: Another way they keep cool is to lower their caloric intake, which conserves energy.

If someone is indeed sweating like a pig, they’re not going to be drenched in sweat. Instead, they’re sweating very little and cooling themselves by marinating in mud.

Is pig iron still being made?

The old-school manufacturing of pig iron is largely relegated to the past, when hot metal was poured into molds resembling suckling pigs and made into ingots for transport. It’s now more common for steel mills to transport the liquid iron to steel plants, though iron ingots are still sold to merchants for steelmaking.

Condensation can collect on a lot of surfaces, and it’s unclear why pig iron became the preferred cliché. But there you have it.

[h/t McGill University]