Having been in the same situation as Dave the only response I've been able to think of on the spur of the moment was "I'm not sure." If I had been able to think more quickly, I might have said "Someone who is not behaving well." But I didn't.

But then, I am the sort of person who, when faced with a toddler having a temper tantrum at its mother, won't give the mother the usual look of disdain that says "Look at her; she can't control her kid!" Instead, I'll fix the toddler with a look and ask "Are you trying to embarrass your mum?" Every time, it stops them in their tracks and causes them to seek Mum's protection from this bearded stranger. And wipes the superior looks from the faces of the disdaining onlookers.

Cheers, Rowan