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).
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 was ended at nones, which also perhaps was incentive to push it forward. 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 was complete by 14c. (the same evolution is in Dutch noen).