Something that is supposititious is false or fake, often pretending to be something it's not.
Calling something supposititious is usually not good because it means it's not real or honest.
Examples:
The supposititious painting was actually a copy.
She told a supposititious story about meeting a famous person.
He tried to sell me a supposititious watch.
The supposititious prince turned out to be a commoner.
The supposititious diamond was just glass.
The supposititious treasure map led to an empty field.