means my man or my guy.
Note: Wangu is often used when you are addressing someone that is at least the same age as you because it is likely to be taken as disrespect when talking to a senior citizen in the Republic of Zimbabwe.
In addition, wangu mostly refers to a male figure. Hence, it's less appropriate to call a woman wangu, but there aren't really hard and fast rules about this. It's a matter of how y'all get along.