πεντεκαιδεκαταῖος
πεντεκαιδεκαταῖος, on the fifteenth day
Morpho works with a static database. If you like, you can compare the output of Morpheus.
Parsed as:
masculine nominative singular of πεντεκαιδεκαταῖος
No checked forms of πεντεκαιδεκαταῖος in the morphology database.