nightmare (n.)

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. 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.

"night-goblin, incubus, oppressed sleep," Old English mare "incubus, nightmare, monster," from mera, mære, from Proto-Germanic *maron "goblin" (source also of Middle Low German mar, Middle Dutch mare, Old High German mara, German Mahr "incubus," Old Norse mara "nightmare, incubus"). This is from PIE *mora- "incubus" (source also of first element in Old Irish Morrigain "demoness of the corpses," literally "queen of the nightmare," also Bulgarian, Serbian mora, Czech mura, Polish zmora "incubus"). Also compare French cauchemar "nightmare," with first element from Old French caucher "to trample" and second element from Germanic.

All this is probably from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death). The word in English now survives only in nightmare (q.v.).