No cap basically mean that their not lying or capping like they did something.
Sometimes ppl write it like this🙅🏽 ♀️🧢or 🚫🧢 different ways

Style with Ease

The use of the phrase "no cap" is meant to convey authenticity and truth. The phrase originated in reference to decorative gold teeth, which can be divided into two distinct varieties: permanent gold teeth (aka "perms") or caps (aka "pullouts"). Whereas caps can be pulled out with ease, perms, as their name suggests, are permanent. They cannot be taken out for a job interview or court date. They are an honest and lasting expression of the owners' realness.

Usually means No lie. Someone usually uses this after saying an unbelievable statement or asking a serious question.

No lie, On God, a definition of telling the truth. Basically no flex.

Slang To not lie or to be telling the truth.

meaning "no lie," it is a misspelling of "no Kapp," where Kapp is a FFZ emote on Twitch.Tv, a sliced picture of the emote Kappa, which is used like "suuuuuuure," so "no Kapp" means no lie