c. 1200, "divine office prescribed for each of the seven canonical hours; the daily service at the canonical hours;" c. 1300, "time of day appointed for prayer, one of the seven canonical hours," from Old French ore, hore "canonical hour; one-twelfth of a day" (sunrise to sunset), from Latin hora "an hour;" poetically "time of year, season," from Greek hōra a word used to indicate any limited time within a year, month, or day (from PIE *yor-a-, from root *yer- "year, season;" see year).
Church sense is oldest in English. Meaning "one of the 24 equal parts of a natural solar day (time from one sunrise to the next), equal hour; definite time of day or night reckoned in equal hours," and that of "one of the 12 equal parts of an artificial day (sunrise to sunset) or night, varying in duration according to the season; definite time of day or night reckoned in unequal hours" are from late 14c. In the Middle Ages the planets were held to rule over the unequal hours. As late as 16c. distinction sometimes was made in English between temporary (unequal) hours and sidereal (equal) ones. Meaning "time of a particular happening; the time for a given activity" (as in hour of death) is mid-14c.
The h- has persisted in this word despite not being pronounced since Roman times. Replaced Old English tid, literally "time" (see tide (n.)) and stund "period of time, point of time, hour," from Proto-Germanic *stundo (compare German Stunde "hour"), which is of uncertain origin. German Uhr likewise is from French.
Greek hora could mean "a season; 'the season' (spring or summer)." In classical times it sometimes meant "a part of the day," such as morning, evening, noon, night. The Greek astronomers apparently borrowed the notion of dividing the day into twelve parts (mentioned in Herodotus) from the Babylonians. Night continued to be divided into four watches (see watch (n.)); but because the amount of daylight changed throughout the year, the hours were not fixed or of equal length.
As a measure of distance ("the distance that can be covered in an hour") it is recorded from 1785. At all hours "at all times" is from early 15c. For small hours (those with low numbers) see wee (adj.).
early 15c., "third of the lesser canonical hours" in churches and religious houses, from Latin sexta (hora), fem. of sextus, ordinal of sex (see six). The office of the sixth hour, originally and properly said at midday. Also "the interval of a sixth in music," etc.
canonical hour, mid-13c., from Old French matines (12c.), from Late Latin matutinas (nominative matutinæ) "morning prayers," originally matutinas vigilias "morning watches," from Latin matutinus "of or in the morning," associated with Matuta, Roman dawn goddess (see manana). Properly a midnight office (occupied by two services, nocturns and lauds) but sometimes celebrated at sunrise. The Old English word was uht-sang, from uhte "daybreak."
mid-12c., non "midday," in exact use, "12 o'clock p.m.," also "midday meal," from Old English non "3 o'clock p.m., the ninth hour from sunrise," also "the canonical hour of nones," from Latin nona hora "ninth hour" of daylight, by Roman and ecclesiastical reckoning about 3 p.m., from nona, fem. singular of nonus "ninth," contracted from *novenos, from novem "nine" (see nine).
The sense shift from "3 p.m." to "12 p.m." began during 12c., and various reasons are given for it, such as unreliability of medieval time-keeping devices and the seasonal elasticity of the hours of daylight in northern regions. In monasteries and on holy days, fasting ended at nones, which perhaps offered another incentive to nudge it up the clock. Or perhaps the sense shift was based on an advance in the customary time of the (secular) midday meal. Whatever the cause, the meaning change from "ninth hour" to "sixth hour" seems to have been complete by 14c. (the same evolution is in Dutch noen).
From 17c. to 19c., noon sometimes also meant "midnight" (the noon of the night).
"earliest canonical hour of the day" (6 a.m.), from Old English prim and Old French prime and directly from Medieval Latin prima "the first service," from Latin prima hora "the first hour" (of the Roman day), from Latin primus "first, the first, first part" (see prime (adj.)). (In classical Latin, the noun uses of the adjective meant "first part, beginning; leading place.")
By extension, "the first division of the day" 6 a.m. to 9 a.m. (early 13c.). The sense of "beginning of a period or course of events" is from late 14c. From the notion of "the period or condition of greatest vigor in life" (by 1530s) comes the specific sense "springtime of human life" (often meaning ages roughly 21 to 28) is from 1590s. Also from 1590s as "that which is best in quality, highest or most perfect state of anything." As "a prime number," by 1530s.