My usual M.O. is to . . .

What’s his M.O.?

I have heard this throughout my life, often on crime shows, but I read it today and realize I had no idea what it stands for.

My first guess was it stood for “method of operations,” or similar. But it actually stands for “modus operandi,” which is basically Latin for “mode of operating.”

So it begs the question: why are we still using the Latin phrase? Why doesn’t M.O. just stand for “mode of operating”?