When someone acts mean, bitchy or out of control. Has no control of themselves, no forethought. Inconsiderate person.
Maxwell Smart: to Agent 99 Is that your default setting or something? Oh, today's Tuesday, I'll punch Max. Oh look, a box of kittens, I think I'll punch Max. I have this piece of bread so now I'm going to punch Max.
"You know she was going to act like that, it's her 'default setting!'