Ex post facto means “after the fact.” A good example is someone trying to change the rules of a game after it’s already begun.
Ex post facto defined: “something that affects things that happened in the past. When a law changes the zoning rules and applies even to zoning decisions that were made in the past, it is an
example of an ex post facto change to the zoning laws.” ADJECTIVE–Your Dictionary.com Such a change is considered retroactive.
Origin of ex post facto
Late Latin, literally, from a thing done afterward.
First Known Use: 1621–Merriam Webster.com