The Brief: No cap means "no lie."

Bluesquiggle
DEEP DIVE

No cap is a synonym for TBH and signifies a moment of honesty. It is used to indicate speaking from the heart.

“Cappin” is also used to mean “lying,” especially in calling out someone else’s dishonesty.

https://twitter.com/AriLennox/status/1093934640627208192

https://twitter.com/queenlilboat/status/1093579044057243648