No cap emphasizes that a claim is sincere or free of exaggeration. Cap can mean a lie or boast, and capping can mean lying or exaggerating.[1]
Usage
“That last piece of chicken was mine, no cap.” The speaker has certified the statement. The empty box is unavailable for questioning.
Hip-hop
The expression is associated with Atlanta hip-hop. Future and Young Thug used it as the title of a track in 2017, after the phrase was already in circulation.