Origin of the name UNDECIMUS.
Etymology of the
Meaning of the baby name UNDECIMUS.
name meaning "eleventh."
... It was a frequent usage,
when the family was numerous, to give to the children a name that should
denote the order of their birth. The first was thus called Primus,
the second Secundus, and so on. Two of these prænomens have
survived. To this day the names of Septimus and Octavius are not
rare. Now, Septimus means "seventh" and Octavius
"eighth." The eleventh child would then take the name
Undecimus, and if it was a girl the form would be Undecima. (Curiosities of Popular Customs and of Rites,
Ceremonies, Observances, and Miscellaneous Antiquities, Walsh, 1897)