The phrase “sweat like a pig” means to sweat profusely. The problem is, pigs sweat very little and roll in the mud to cool down. They do so because a pig’s sweat glands are ineffective at keeping them cool. So where did the phrase originate? It turns out that it refers to the smelting process. Smelting produces metal from ore, in this instance, iron ore which is smelted into what’s called pig iron. The name comes from the shape of the iron, which vaguely resembles a group of piglets suckling on a sow. Before the iron can be handled, it has to cool. Smelters know this when the “pigs” in the iron “sweat,” which is caused when the air around the iron reaches the dew point and droplets form on the metal’s surface.
Another language note is that the word pig solely refers to young swine. So, swine is the proper term in referring to this well-known farm animal.
While we’re on the subject of pigs, another misconception is that they are filthy animals. In fact, pigs are among the cleanest animals on the farm. Pigs are also smarter and more easily trained than dogs or cats [sources: The Humane Society, PBS Nature].