Old Saying

That old saying, sweating like a pig, I used to laugh at that. I thought pigs were gross. They would be all muddy and sweaty and just gross. That is a mis characterization of a pig. Pigs do like the mud but it’s because they do not sweat. The mud keeps them cool when it is hot out. I wonder where someone got that old saying since pigs don’t sweat. It’s a general wondering on my end. Anyone know the answer??