My first instinct was that this phrase is racist, especially with you saying it was used in an ex-British southern African colony.

This was the best I could find as to its origins:

https://boards.straightdope.com/t/where-did-thats-mighty-whi...

Relax, the past is a foreign country, they do things differently there.

And so's the future - probably next century we'll be looked down upon as sad sacks of shit who didn't have the decency to get our weights into clean silicon.