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.
I would only get perms because people who wear pullouts are fake, no cap.
A phrase that is akin to “it is true that” or “I think that”, “no cap” is often used to describe an opinionated statement or a statement that is believed to be true. Stemming from the verb capping (basically, “to lie”), it quite literally means “no lie.” Because of this, no cap can be used to describe something that is not thought to be true by the speaker’s audience. No cap can be used to describe factual statements, but due to its slang nature, it is most often used in an exclamatory manner.
“No cap, this pizza is amazing!”
“I think I actually like Ty, no cap.”
“Dude, I’m serious about this— no cap.”