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

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.

Not lying/ not tripping/ telling the truth periodt

People that say no cap are the ones that are capping the most.

No lie or not lying

(from skater slang)
means that something is the truth or that you're not lying.
the phrase was ruined by tiktokers and just 12 year old kids using it in general

When u don’t lie to ur homies