These Janus words are also called auto-antonym, or a word that means the opposite of itself. If I say, there are four people at a dinner party. By midnight, two people had left. How many people are left at the party? No, it is not a Math question.
The woodcutter swung the axe down hard in order to cleave the log into two even pieces. Young beaver pups cleave to their mother in the water until they are strong enough to swim on their own.
Can we think of a few more contronyms while at it?
Nice wasn’t as nice as it is now.