In slang, cap means a lie or something that isn’t true. When someone says that’s cap, they’re calling something fake, exaggerated, or dishonest. The opposite is no cap, which means someone is being truthful or serious. It’s commonly used in texting, social media, and everyday conversation, especially among younger people. For example, “He said he finished the work in 5 minutes—that’s cap” means the speaker doesn’t believe it’s true.