https://www.dictionary.com/e/slang/no-cap/
In Black slang, to cap about something is to brag, to exaggerate, or to lie about it. This meaning of cap dates back to the early 1900s.
History lesson: In the 1940s, according to Greens Dictionary of Slang, to cap is evidenced as slang meaning to surpass, connected to the ritualized insults of capping (1960s). These terms appear to be rooted in the sense of cap as top or upper limit.
So, no cap has the sense of no lie, no joke, for real, or not bragging. The expression is closely associated with slang in Atlanta-area hip-hop. It appears on Twitter by at least 2012, though it was almost certainly in use in spoken English before then.
BtCV
easy's gettin' harder every day