It's possible that "choo" could have a specific meaning in a particular African language or dialect, but without more context, it's impossible to determine its meaning.
If you can provide more information about the context in which you heard the word "choo," it might be possible to pinpoint the language and its meaning. For example:
* Where did you hear it? (What country or region?)
* What was the situation? (A conversation, a song, a movie, etc.)
* What other words were used around it?
With more information, we might be able to help you understand the meaning of "choo".