When I say I'll tan his hide when I see him is it the mild way to say I'll kick his ass? Who would use it (I mean which education he'd have or social class he would belong to?) and how angry would that person be?
Usually this phrase is used when a parent is threatening to spank a child: "You get your chores done or I'll tan your hide!" Although any type or class of person could say it, it's generally a phrase associated with people in the South or from rural areas. (It's something Aunt Polly would say to Tom Sawyer, if that's any help.)
Tanning is also a process for turning hides into leather. I think it has something to do with getting the leather a nice tan color. If we look at the examples above you will see the connection. When you spank someone, the skin will turn a different color.