Origin and history of twi-night

late Old English niht (West Saxon neaht, Anglian næht, neht) "the dark part of a day; the night as a unit of time; darkness," also "absence of spiritual illumination, moral darkness, ignorance," from Proto-Germanic *nahts (source also of Old Saxon and Old High German naht, Old Frisian and Dutch nacht, German Nacht, Old Norse natt, Gothic nahts).

The Germanic words are from PIE *nekwt- "night" (source also of Greek nyx "a night," Latin nox, Old Irish nochd, Sanskrit naktam "at night," Lithuanian naktis "night," Old Church Slavonic nosti, Russian noch', Welsh henoid "tonight"), according to Watkins, probably from a verbal root *neg- "to be dark, be night." For spelling with -gh- see fight. The vowel indicates that the modern English word derives from oblique cases (genitive nihte, dative niht).

The fact that the Aryans have a common name for night, but not for day (q.v.), is due to the fact that they reckoned by nights. [Weekley]

Thus in Old English combinations night was "the night before (a certain day or feast day);" compare German Weihnachten "Christmas," literally "holy night." In early times, the day was held to begin at sunset, so Old English monanniht "Monday night" was the night before Monday, or what we would call Sunday night; so saeterniht "Friday night." Thomas à Becket's Wednes-night (Old English Wodnes-niht) would be our Tuesday night. The Greeks, by contrast, counted their days by mornings.

To work nights preserves the Old English genitive of time. Night soil "excrement" (1770) is so called because it was removed (from cesspools, etc.) after dark. Night train is attested from 1838; night-school from 1520s; night-life "habitual nocturnal carousing" is attested from 1852.

"light from the sky when the sun is below the horizon at morning and evening," c. 1400 (late 14c. as twilighting), a compound of twi- + light (n.) Cognate with Middle Flemish twilicht, Dutch tweelicht (16c.), Middle High German twelicht, German zwielicht. Glossing Latin crepusculum.

The connotation of twi- in this word is unclear, but it appears more likely to refer to "half" light than to twilight's occurring twice a day. Compare also Sanskrit samdhya "twilight," literally "a holding together, junction," Middle High German zwischerliecht, literally "tweenlight."

Originally and most commonly in English with reference to evening twilight but occasionally used of morning twilight (the specific sense attested by mid-15c.).

In reference to any faint light or partial darkness from 1660s. Figurative extension is by c. 1600, as "intermediate position or period," also "indistinct medium of perception, state of hazy illumination." As an adjective, "belonging to or pertaining to twilight" (1620s).

Twilight zone is from 1901 in a literal sense, a part of the sky lit by twilight; from 1909 in extended senses in references to topics or cases where authority or behavior is unclear. The U.S. TV series of that name is from 1959. In the 1909 novel "In the Twilight Zone," the reference is to mulatto heritage ("She was in the twilight zone between the races where each might claim her ..."). James Russell Lowell (1889) has twi-life "life marked by indistinct consciousness or awareness."