What is the origin of the idiom "rosy about the gills" and when to use it?

The idiom "rosy about the gills" likely originates from the idea that gills in fish are a vital part of their respiratory system, and when fish are healthy, their gills are often a rosy or pink color. Over time, this imagery has been extended to describe the healthy and glowing appearance of people, creating the idiomatic expression we use today to signify someone looking well or in good spirits.