Someone who is hard and up your ass one minute, but the next, they are soft and can barely get in your head.
It's hard to hate Isaac forever because even though he's a dick, he's an inconsistent dick, and he'll apologize just as soon as he's done being mean to you.
An IPD (Inconsistent Physics Definintion) is when an entity or object is taken to a different universe than their original universe, but has different laws the entity/object cannot adapt to.
John: Uhh... Bob? Do you know why my mug is so small?
Bob: I think it has experienced Inconsistent Physics Definitions.