Capping basically means that you are kidding or joking around so that would mean that “No cap” means that you aren’t.
Brief History: The term originates from the Twitch emote “kappa” which means that you are joking. Instead of when you aren’t kidding saying “No kappa”, it was shortened down to “No kap” and then interpreted by mainstream audiences as “No cap”.
Aye you look like shit, no cap.
Bro I’m not cappin when I tell you this.