What is the origin of the idiom "near the knuckle" and when to use it?

The idiom "near the knuckle" is often used to describe a remark, joke, or comment that is very close to being offensive, rude, or inappropriate but does not quite cross that line. The exact origin of this expression is not well-documented, but it is believed to be a British phrase that likely developed from the idea of a knife edge or the edge of a blade.