An absurdity is something ridiculously foolish. The U.S. declaring war on Fiji would be an absurdity.
This is the noun form of the adjective absurd, which means silly, hard-to-believe, and kind of crazy. Unless it's Halloween, it would be an absurdity to show up to school in a chicken suit. If you teacher taught math class with a fake French accent, that would also be an absurdity. A lot of comedies are full of absurdities, because an absurdity can be funny. If you're good at thinking of absurdities, maybe you should be a comedy writer.