To me, if that's a stranger, then why put any weight or meaning in anything they're saying? Just ignore it and move on. Or if you want to add an insult on the out, say something like, "I'm sorry, nothingness can't possibly insult me." Or whatever else that doesn't directly insult them.
But if it's someone you know well, then it can have any number of meanings, they're usually either joking around, or just in a bad mood for whatever reason. Be a better person and either be patient with them or be playful and tease back if that's what they're doing. That's just me for your question though... And it is a pretty broad question...
Have an awesome day!