No cap is a slang expression, especially used in black lingo as an alternative for “no lies”.
It is utilized in situations, when the told story is hard to believe.
Cap in black slang has been used as a synonym for exaggeration since the beginning of the last century and has remained in the language as such since then.
In the 1940’s to cap was used as the verb to surpass, possibly in the sense that cap is signifying upper limit.
No cap is nowadays a slang expression for no lies, especially in the area of Atlanta, and the hip-hop that’s coming from there.
The term has been first seen on Twitter in 2012, though in spoken language it has obviously been present for a long while.
The phrase got popularized in 2017, when Young Thug and Future released the track “No Cap” together, where the two rappers brag about the wealth and articles of luxury they’ve amassed, with no cap included.
No cap is more widespread than ever, though its links to hip-hop and black culture are still strong, so it is mostly seen on the Twitter feed of black users and those, associated with Atlanta rap.