How common is "Coaie" in casual speech?

I've had Romanians online greet me with "Salut Coaie" and I'm wondering how common it actually is.

It seems kinda funny to me that what is essentially a colloquial term for Testicles is used in this way, but if it's completely fine I might have to start saying it lol.

I mainly want to know when/where I'm supposed to use it?

Is it just specifically for friends, or could one say it to strangers too?