A cynic is someone who believes that humans are selfish and that they only do something if it will benefit themselves. Cynics criticize acts of kindness and will probably tease you if you help an old lady cross the street.
Maybe you know a cynic or two. They like to criticize people and situations, especially if they think there’s a deeper motivation behind the more obvious one. Let’s say a friend borrows your car, and then he shows up later covered in grease and offers you a lollipop. If you’re a cynic, you’d suspect that your friend crashed your car and is trying to distract you with the delicious and sweet lollipop. And you might be right.