I am very sure we have all heard the idiom"sweat like a pig".But have you ever thought of its origin?.This idiom actually creates a linguistic problem,a result of the fact that pigs hardly sweat because they possess few sweat glands in their skin which explains why they are always prefer to wallow in cool water or muds because it helps cool their body temperature.This is equivalent to the cooling effect that results from sweating.Instead this idiom alludes to iron smelting process. Smelting is the process of extracting a metal from its ore(in this case iron from its ore).In the process of smelting an iron ore,it is first converted to a brittle form of iron called pig iron which is of little use because of its traits.This brittle products are called pig iron because of their semblance to a group of piglets attached to their mother.This pig iron is heated to extreme temperature and the resulting liquid metal is poured into a mold.Before pouring the liquid metal into a mold,the smelter allows the liquid metal to cool because a hot liquid metal is liable to spilling which will burn whatever it comes in contact with. But how does the smelter knows when the liquid metal has cooled?.It is because the pig iron "sweat" after coolingi.e droplets of water appear on its surface because the surrounding air condenses to droplets after attaining its dew point.So the idiom is actually referencing this pig iron not actual pigs. CONCLUSION:Pigs don't sweat. Pig iron do�

...and hence why "sweating like a christmas goat" or "sweating like a thanksgiving turkey" is more appropriate.

Sweating like a pig is inaccurate but used mostly to emphasize the difficulty of a physical job done. I like it and will use it regardless. It's a saying people have grown to accept.