A recent edition of the TV programme Countdown announced undinal as a valid English word, being an adjective derived from undine, a water spirit. But it does not appear in my Oxford English dictionary, nor any similar Latin dictionaries (except as unda - a wave), nor the SCrabble dictionary or any other according to Wordnik (this page). Etymology apparently stems from the 16th or 17th Century Latin coinage.
Oh, fun! It doesn't surprise me that something might be missing from the Scrabble dictionaries. Traditionally, the Official Scrabble Players Dictionary pulled from just "five in-print collegiate dictionaries, namely The Random House College Dictionary (1968), The American Heritage Dictionary of the English Language (1969), Webster's New World Dictionary (1970), Merriam-Webster's Collegiate Dictionary (1973) and Funk & Wagnalls (1973)" (quoting https://en.wikipedia.org/w/index.php?title=Official_Scrabble_Players_Dictionary&oldid=698206686).
So I looked up undine on an online version of the OED (subscription only, sadly). At the bottom of the entry, it has a "Draft additions 1993" section which has information about undinal--it references the 1891 Century Dictionary definition--which brings us right back to the Century definition here on this Wordnik page.