Deceit is purposeful falsehood. If you want to keep your true love true to you, avoid any acts of deceit.
Deceit comes from the Latin for "cheat." When you deceive someone, you give them a false impression, mislead them so as to get away with something bad. If a person speaks in a way that gives a false impression, we call the way he speaks "deceptive." If someone pretends to be someone else, that’s an act of deceit, like when a thief pretends to be a valet parking attendant and takes off with your car.