Facial stubble; a man's beard at the end of the day.
1) Peter has a very heavy beard -- even though he shaves every morning, he gets a five o'clock shadow by lunchtime!
The typical American workday ends at 'five o'clock' in the afternoon. A 'shadow' is a patch of darkness, or a hint of the presence of something. After spending a full day at work for eight or more hours, many men have a noticeable growth of facial hair, which is dark like a 'shadow' and hints at the beard that would grow if left unshaven.