Undecennary \Un`de*cen"na*ry\, 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. --E.
Stiles.
[1913 Webster]