late 14c. in reference to the Roman calendar, "ninth day (by inclusive reckoning) before the ides of each month" (7th of March, May, July, October, 5th of other months), from Old French nones and directly from Latin nonæ (accusative nonas), fem. plural of nonus "ninth," contracted from *novenos, from novem "nine" (see nine).
The ecclesiastical sense of "daily office said originally at the ninth hour of the day" is from 1709; originally fixed at ninth hour from sunrise, hence about 3 p.m. (now usually somewhat earlier), from Latin nona (hora) "ninth (hour)," from fem. plural of nonus "ninth." It was also used from c. 1300 in a general sense of "midday" (see noon). For for the nones, see nonce.