You know when you've done something really wrong, and the person who gets you in trouble has that unforgiving look on his face? The best word for that look is stern, meaning "strict" or "severe."
Stern, strict, severe, harsh, unforgiving — they all more or less mean the same thing, which is very tough and exacting, with a little helping of seriousness thrown in for good measure. What does this have to do with the stern of a boat, also known as the rear area? Um, nothing? But it's good to know about that meaning too, since it could come in handy some day when you're giving directions on a ship.