Noun. Metaphoric phrase most commonly used to describe a highly unusual or unforeseen event or occurrence. Can be extended to describe people who are not as they first appear, or when a person acts out of character.
Originally derived from a quotation in '90s cartoon television show "The Tick" in which the eponymous hero, upon seeing a pig on a rooftop, exclaims "Roof Pig! Most unexpected!".
Wow! Saints won the Superbowl? That's one heck of a roof pig!
She was acting so strangely the other night - a total roof pig.