What is the origin of the idiom "on the pig's back" and when to use it?

The idiom "on the pig's back" is a literal translation of the Irish phrase "ar mhuin na muice," which dates back to the 17th century. In Irish, it means "well off" or "in luck," and in English, it is used to convey being in a prosperous or happy state, or doing really well. The phrase is used to describe a successful or established situation.