"long, loose outer garment reaching almost to the floor, worn by men or women over other dress," late 13c., from Old French robe "long, loose outer garment" (12c.), from a Germanic source (compare Old High German rouba "vestments"), from West Germanic *raubo "booty" (cognate with Old High German roub "robbery, breakage"), which also yielded rob (v.).
Presumably the notion is of fine garments taken from an enemy as spoil, and the Old French word had a secondary sense of "plunder, booty," while Germanic cognates had both senses; as in Old English reaf "plunder, booty, spoil; garment, armor, vestment."
The meaning "dressing gown" is from 1854; such extended senses often appear first in French, e.g. robe de chambre "dressing gown," robe de nuit "nightgown." From c. 1300 in reference to official vestments and thus indicative of position or membership in a religious order, guild, etc.; metonymic sense of The Robe for "the legal profession" is attested from 1640s.
"to clothe," especially magnificently and ceremonially, c. 1300 (implied in robed), from robe (n.). Related: Robing; robery.
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.