“Cap” can mean a lie or exaggeration. “No cap” signals that the speaker means what they just said.

“That was the best set of the night, no cap.”
Calling someone’s story “cap” challenges its truth. “You’re capping” makes that challenge more direct.
“Cap” can mean a lie or exaggeration. “No cap” signals that the speaker means what they just said.

“That was the best set of the night, no cap.”
Calling someone’s story “cap” challenges its truth. “You’re capping” makes that challenge more direct.