A shirt named for the hour it is worn
A nightshirt is a long, loose, shirt-shaped garment made for sleeping. Traditional cuts fall to the knee or below and leave part of the legs uncovered. In later American use the same word can also mean a slightly elongated shirt worn as loungewear, sometimes with pajama trousers. For a long stretch of modern English it was treated as the male counterpart of a nightgown. That split has loosened: shops now sell the long shirt as unisex sleepwear for anyone who prefers one piece to a matching set.
The origin of the word nightshirt is unusually easy to see. It is night plus shirt, joined inside English rather than borrowed as a finished term from French or Latin. Historical dictionaries place the noun in the mid-1600s. The Oxford English Dictionary gives a letter of 1657 by R. Verney as its earliest evidence; Merriam-Webster independently lists the same year. Collins, using a later editorial cut, dates the solid modern spelling to about 1840–50. The clash is over when the written compound settles, not over what the two halves mean.
English joinery, not a loanword
English already had both pieces for centuries before anyone needed a special name for bedwear. Night is native. Late Old English niht — West Saxon neaht, Anglian næht — named the dark part of the day, darkness itself, and, by metaphor, moral or spiritual ignorance. It comes from Proto-Germanic *nahts, and further from a widespread Indo-European night-word, reconstructed as *nekwt- or *nókʷts. Close kin include German Nacht, Dutch nacht, Gothic nahts, Latin nox, Greek nyx, Lithuanian naktis, and Old Church Slavonic nosti. The modern spelling with gh is a Middle English scribal habit for an older guttural, the same device seen in fight. The vowel of present-day night reflects Old English oblique forms such as nihte and niht rather than the nominative.
Shirt is Germanic as well. Middle English shirt and shirte go back to Old English scyrte, a short garment whose exact everyday sense in Old English is poorly attested. The historical dictionaries note that the word appears there mainly as a gloss whose Latin original may have been misunderstood. The deeper Germanic form is a short garment, *skurtijǭ, related to the adjective for short and, by a widely accepted if not ironclad link, to a root *sker- meaning to cut: clothing thought of as a cut length of cloth. Shirt is a doublet of skirt. The sh- form is the English development; the sk- form came back in through Old Norse skyrta. Scandinavian languages keep the shirt sense: Icelandic skyrta, Swedish skjorta, Danish skjorte. In German and Dutch the related words Schürze or Schurz and schort drifted toward apron.
The compound is therefore English joinery of two inherited stems. German Nachthemd and Dutch nachthemd look parallel, but they are not the source of the English word. They pair their own night-word with Hemd, a shirt or chemise from a different Germanic line, related to Old English hemeþe rather than to scyrte.
From day-shirt to a garment kept for bed
The object has a messier history than the label. Specialized nightclothes were not the medieval default. People often slept unclothed or in whatever they had worn next to the skin during the day. Costume historians describe a change from the sixteenth century: men who could afford it began to keep a separate shirt for bed. Period English spoke of embroidered or wrought night-shirts among the wealthy. By the nineteenth century the typical nightshirt looked like a day-shirt stretched longer, with a loose turned-down collar, sometimes made from cheap or worn cloth because the garment was not meant to be seen in public.
Nightgown is older as a written compound, recorded from about 1400, and for a long time it could mean a loose gown put on at night by either sex, or even a garment worn out of the house. Nightshirt carved out a more specific image: shirtlike rather than flowing, and for a stretch of modern English especially tied to men and boys. The adjective night-shirted, recorded from 1856, shows the hyphenated form still current in the Victorian period, sitting beside the later fused spelling.
Two-piece pajamas then displaced the long nightshirt for many wearers. The newer set was adapted in British India from Persian pāy-jāma, a name for a leg garment, plus a shirt, and became Western sleepwear in the late nineteenth century. Mass-produced pajama sets in American shops after that made the older one-piece look old-fashioned. The nightshirt did not disappear. It survived as a nostalgia cut, a summer alternative, and later as unisex sleepwear and thigh-length loungewear.
What the two halves still mark
The useful residue is morphological honesty. Night still marks time of use, as it does in nightcap, nightgown, and night-school. Shirt still marks cut: a T-shaped upper garment, collared or not, here simply lengthened. The pairing never needed a foreign intermediary. If you want one date to carry away, take 1657 as the first firmly cited English noun, and keep in mind that some desk dictionaries lag by two centuries because they are dating the fused spelling, not the idea of a shirt reserved for night.