Of course, every word was made up at one time, but as long as you're not making it up for a particular puzzle, that's OK. It really all depends on the situation (publisher, day of the week, type of puzzle). A few editors would insist on it being a "real" word, meaning actually in dictionaries, but many like to use such entries. Argument against: although it's been used in respected publications, it has so few occurrences that it's likely no one who sees this puzzle will have ever seen the word in print. Argument for: its meaning is obvious (unless you don't know who Eeyore is), it's cute, and it's a debut word. You get to decide whether to try it, and the editor gets to decide whether to OK it. Good luck!
