If someone handed you a dollar bill and told you it was fictitious, you might be slightly confused. You can feel it between your fingers, so how could it be fictitious?
That’s because these days, the word fictitious is typically used to mean imaginary. The Loch Ness monster, for instance, is a fictitious creature (or so we think). And back in the early 17th century when the term first appeared in print, that definition was common—but it wasn’t the only one. Fictitious could also describe a counterfeit item; anything fake or imitated. The noun fiction is even older, having first shown up in writing during the late 15th century. It, too, could refer to a made-up concept or a counterfeit creation. Both words are derived from the Latin verb fingere, meaning “to form” or “to fashion.” It’s the same place we get feign.
By the late 16th century, people had started using fiction the way we usually do—to describe literature based on imaginary events and characters, rather than fact. And when fictional arrived on the scene in the mid-1800s, it was basically the result of people turning this sense of fiction into an adjective. That correlation is even stronger these days, though our meaning of fiction has expanded to encompass more modern media, like TV shows and movies. If someone mentions a “fictional character,” they’re probably talking about a character from a book, film, or some other work of fiction.
As Grammarist explains, the contemporary usage of fictitious is a bit broader than that. It can describe a movie character or another element of fiction, but it can also be used to modify anything else imaginary or fabricated—especially if there’s deceit involved. If someone concocts an alibi to place them far from the scene of a crime they committed, you’d be more likely to consider it a fictitious alibi than a fictional one.
That said, as is so often the case with the English language, the distinctions between fictional and fictitious are more guidelines than concrete rules. If you want to describe an alibi as fictional, nobody’s stopping you.