noon (n.)

mid-12c., non "midday, 12 o'clock p.m., midday meal," from Old English non "3 o'clock p.m., the ninth hour," also "the canonical hour of nones," from Latin nona hora "ninth hour" of daylight, by Roman reckoning about 3 p.m., from nona, fem. singular of nonus "ninth" (see nones). Sense shift from "3 p.m." to "12 p.m." began during 12c., when time of Church prayers shifted from ninth hour to sixth hour, or perhaps because the customary time of the midday meal shifted, or both. The shift was complete by 14c. (same evolution in Dutch noen).