What is the origin of the idiom "five o'clock shadow" and when to use it?

The origin of the phrase is uncertain, but it is believed to have emerged in the early 20th century. One theory is that it refers to the shadows that men's beards would cast on their faces as the sun began to set around 5 p.m. Another theory is that it comes from the practice of businessmen in the 1920s and 1930s who would leave work at 5 p.m. with their faces unshaven to give the impression that they had been working hard all day. The idiom is commonly used in informal settings and is often associated with a casual or rugged look.