No cap
Etymology
From African American Vernacular English, where "cap" has long meant to lie, boast, or exaggerate (and "capping" the act of doing so), a usage documented for decades. "No cap" spread into wider slang in the late 2010s, helped by hip-hop, including the 2018 Migos track "No Cap."
phrase · /noh KAP/
No lie; for real — an assurance that what you’re saying is true.
What it names
It names an explicit truth-marker you attach to a claim to head off disbelief — "no cap, I finished the whole thing in a day." Its opposite, "cap," calls out a lie or exaggeration.
Example
No cap, that’s the best meal I’ve had all year.
Related idea
The blunt cousin of deadass (AAVE for "seriously"); both vouch for sincerity, but "no cap" specifically denies a lie.