To resent something is to feel anger or bitterness toward it. You might resent someone who has treated you poorly.
To resent is a strong, negative feeling. You may resent the accusation that you were stealing cookies, or when a teacher yelled at you for whispering, even though everyone else was too. You might resent a friend who has more money or friends than you. Lots of people resent celebrities because they're famous and wealthy. If your friend borrowed a sweater and didn’t return it, you would probably resent it. Resenting is the opposite of being grateful.