Basically, 21 in rap can mean 2 things so you better read this article thoroughly to know how and when to use it. 21 is a very common rap slang that you might find being used in urban communities or rap singles.
The first meaning of 21 is to refer to someone as an adult. 21 is often the legal age for every state in the US. If you are 21, you can drive, drink, go to bars, and many more. It is when you can start enjoying life. So you might hear something like this:
“You 21. You should be your man and start making your own decisions.”
For the second usage of “21,” we couldn’t find the origin of the slang. Another use case of 21 is when you want to say something like “stop it” or “quit it.” You might someone speak:
“21, bro! You are pushing your mom to the limit.”