To cheat is to use sneaky, unfair methods to get something you want. No one's going to want to play with you if you always cheat at mini golf.
Some people cheat at cards, and others cheat during math tests by copying from the student beside them. In either case, they're trying to succeed using underhanded tricks. You can refer to someone who does this as a cheat as well. Cheat comes from the Old French escheat, a legal term for the state taking someone's property when he or she dies without heirs. This practice was resented, and it came to mean "confiscate unfairly."