The Collaborative International Dictionary
Nundinal \Nun"di*nal\, Nundinary \Nun"di*na*ry\, a. [L. nundinalis, nundinarius, fr. nundinae the market day, the weekly market, prop., the ninth day, fr. nundinus belonging to nine days; novem nine + dies day: cf. F. nundinal.] Of or pertaining to a fair, or to a market day.
Nundinal letter, among the Romans, one of the first eight letters of the alphabet, which were repeated successively from the first to the last day of the year. One of these always expressed the market day, which returned every nine days (every eight days by our reckoning).
Wiktionary
n. In Ancient Rome, one of the first eight letters of the alphabet, which were repeated successively from the first to the last day of the year. One of these always expressed the market day, which returned every nine days (every eight days by our reckoning).