What is the origin of the idiom "pound for pound" and when to use it?

The origin of the idiom "pound for pound" can be traced back to combat sports, specifically boxing. In boxing, fighters are categorized into weight classes, and rankings are determined based on their performance within those classes. It implies that if the fighters were the same weight, the one being referred to would still possess exceptional skills or abilities. Figuratively, the idiom is primarily used to apply to various contexts, highlighting the hidden or underlying qualities of something that may appear significantly different on the surface.