Source : Webster's Revised Unabridged Dictionary (1913)

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.

         An undecennary account laid before Parliament. --E.
                                                  Stiles.