a. Obs. rare–1. [An irregular formation from L. tredecim thirteen.] Intended to mean: Of or pertaining to the number thirteen.
(By the form, it ought to mean ‘Of thirteen years.’)
1783. Robertson, Hist. Amer. (ed. 4), III. 385. Computed … first by what he calls a tridecennary progression of days from one to thirteen,… and then by a septenary progression of days from one to seven, making in all twenty.
