Middle English sight, from Old English sihð, gesiht, gesihð "thing seen; power or faculty of sight; aspect; vision; apparition," from Proto-Germanic *sekh(w)- (source also of Danish sigte, Swedish sigt, Middle Dutch sicht, Dutch zicht, Old High German siht, German Sicht, Gesicht), stem that also yielded Old English seon (see see (v.)), with noun suffix -th (2), later -t (14c.).
The meaning "perception or apprehension by means of the eyes" is from early 13c. The meaning "device on a firearm to assist in aiming" is from 1580s. A "show" of something, hence, colloquially, "a great many; a lot," (late 14c.). As "something that calls forth glances of shock, amusement, etc., a shocking spectacle," by 1862.
Sight for sore eyes "welcome visitor" is attested from 1738; sight unseen (adv.) "without previous inspection" is from 1892. Sight gag is attested by 1944. To feel or know something at first sight is from c. 1300. From the firearm aiming sense come in (one's) sights; have (one's) sights set on something. To keep out of sight is from late 14c.; to be out of (someone's) sight is from c. 1400.
nursery talk, "good-night," 1896; form nighty-night is attested from 1876.
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." 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.