Deceit means the practice of deceiving others. To deceive someone means to mislead him or her. When you give a mistaken impression about something, to another person, you deceive that person. Deceit makes someone to believe in something false and it can be deliberately done by someone in order to take someone else for a ride. Gullible people, fall prey to such deceitful people. To save yourself from deceit, you should know how to judge others and see through their intentions. George Orwell had once remarked, "During the times of universal deceit, telling the truth will be a revolutionary act."
Deceitful people hardly have any ethics and deceive others with a purpose in mind. In the words of Noel Coward, "Integrity is the lifeblood of democracy. Deceit is a poison in its veins." Deceitful people can never live peacefully. One or the other day, they get paid in kind by the Almighty, himself.