A falsity is either a straight-up lie or the fact that something isn’t true. If you fake cry to try and get out of trouble, the falsity of your emotions will get you busted.
Falsity is a word for statements that are false in the sense of being incorrect, untrue, or even dishonest. If you tell a lie, you've told a falsity, which can also be called an untruth or falsehood. Also, falsity can mean the general state of falseness. Con men and other liars often operate in falsity. A spy must use falsity to fool people. The opposite of falsity — or falseness — is truth.