تعريف ومعنى "hairy at the [heel]"

What is the origin of the idiom "hairy at the heel " and when to use it?

The idiom "hairy at the heel" dates back to at least the 17th century. It originally referred to horses that appeared healthy except for fur or hairy growths around their heels, which indicated underlying problems. Today, it is used to describe a person who has bad manners and not behave well in social situations.