This has been on mind for days now, and research hasn't shed any light on it. So I turn to the great minds of JWD to rescue me!
Why on earth does the word "penultimate" exist? It means "next to last", so if you're reading a 20-chapter book, the 19th chapter is the penultimate chapter. It doesn't mean "super-ultimate", it doesn't have anything to do with the common definition of "ultimate". (Though they may be related etymologically.)
Why would we want a word like this? What do you use it for? Has it been used in some great literary work, or does it have a use poetically?
You can't really use it in normal conversation, since it's likely to be misunderstood. But even if you could, why would you?
Just for kicks, there's even "antepenultimate", meaning "coming before the next to the last in a series". When would you EVER need this word?!?