What does sweat like a pig mean and why?

The phrase "Sweating like a Pig" means sweating profusely. This sounds illogical, as pigs (the animal) have ineffective sweat glands, but the term is derived from the iron smelting process. After pouring into runners in sand, it is allowed to cool and is seen as resembling 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 moved in safety. it just means that when a pig is hot it rolls in mud.