Why do I need to know mon vieux or ma vieille?

Because it's a widely used casual term of endearment.

What does it mean?

As most people with a fairly basic grasp of French will know vieux (or vieille if you are talking about a woman or a feminine object) means old. So calling someone mon veiux is calling them my old fella.

But in fact this is a common term of endearment similar to calling someone mate or pal.

And it's regularly used too. While its closest English translation – old man or old boy – is now very old fashioned and not often heard outside World War II movies, the French equivalent is still common.

Salut, mon vieux, ça va? – Hiya pal, how's it going?

Merci, mon vieux, c'est mon vin préféré – Thanks mate, that wine is my favourite

As is its female equivalent ma vieille.

Merci d'avoir fait le ménage ma vieille, les tapis sont nickels – Thanks for doing the housework, love, those rugs are spotless

Ne t'inquiète pas ma vieille, il va revenir – Don't worry mate, he'll be back

For more French terms of endearment that sound rather insulting, click here.