means that you’re not lying and are being serious. originated from the twitch emote Kapp and non twitch users started using it as “Cap” after hearing twitch users started saying it in public.
No cap means: Keeping it real, not lying or exaggerating.
The term comes as a reference to having sex with no condom/cap living the experience 100% and not feeling like it wasn't the real thing.
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”.