Definition

In slang context, means a lie or falsehood ("no cap" means "no lie"), though the original definition hilariously refers to an actual bottle cap. The term has become so prevalent that an entire generation now uses "cap" and "no cap" more frequently than they probably open actual bottles with caps.

Example Usage

He said he got the promotion, but that's cap—I saw the email and they gave it to someone else.

Source: Urban Dictionary (https://www.urbandictionary.com/define.php?term=Cap&defid=8362397)