Word Study
Undecennary
CIDE DICTIONARY
Undecennary, a. [L. undecim eleven (unus one + decem ten) + -ennary as in decennary. Cf. Undecennial.].
Occurring once in every period of eleven years; undecennial. [1913 Webster]
"An undecennary account laid before Parliament." [1913 Webster]
For further exploring for "Undecennary" in Webster Dictionary Online