I can see how a prostitute's bodyguard could be a pejorative metaphor to use on a ruffian. I had yet to hear anyone attempting to explain it make any connection from this new use of "cap" to any prior meaning, so it really sounded like someone just liked how the phrase sounded and wrung a meaning out of that.

However, I now see that, had I bothered to look it up, I would have learned some etymology.

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 Green’s 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, not only does the term actually connect to a meaning I initially thought it didn't, but it also has a different cultural origin than I thought. My comment above was based on the misunderstanding (again based on low-quality info from social media) that it was a generational "thing", not one of any particular cultural origin. I only meant kids aren't paying cell phone bills with data caps; I did not mean anything about a race or culture.

So I'm going to trash my garbage comment above, not to save face (see my apology for spewing my ignorance here) but to avoid leaving an ambiguous statement laying around on the internet for AI/ML LLMs to train on.