A date that does two jobs
In everyday English a birthday is the yearly return of the date someone was born. It is also, without a change of form, the natal day itself — the calendar fact of the birth — and the gathering held for either of those. The same noun stretches to things that were never in a crib: a newspaper can mark a hundredth birthday, a republic a founding one. People use it for a personal anniversary first, and for origin-days of institutions by analogy.
The origin of the word birthday looks almost too plain, which is why the real story is not the two halves you can still hear, but how English rebuilt the compound and whom it first named.
A Germanic pair, not a loan
The spelling we use is a transparent English compound of birth and day. Historical dictionaries place the Middle English forms — birthdai, birtheday, and close variants — in the later fourteenth century, often in a window around 1350–1400. The Oxford English Dictionary analyses the modern noun as formed inside English from those two nouns. That is accurate for the word as we write it. It is incomplete as a history of the idea.
Old English already had a matching compound, ġebyrddæġ, also cited in some accounts as byrddæg. It joined ġebyrd, a noun covering birth, descent, offspring, and even fate, with dæg, day. That compound is traced to Proto-West Germanic *gaburdidag, with an Old High German relative giburtitag. English did not wait for French or Latin to invent a name for the occasion. A West Germanic stack of birth-word plus day-word was already in the language.
What changed was the first half. The simplex birth of modern English is not a straight survival of ġebyrd. From about 1200 the noun for the fact of being born — and soon for childbirth, a child, and lineage — came from a Scandinavian source, Old Norse burðr and its kin, and largely replaced the native Old English form. Middle English then put that newer first element onto day, yielding the compound that settled as birthday. The ge- prefix of ġebyrd fell away with the old simplex. In that sense the modern word is both inherited and remade: same two notions, a first element recast under Norse pressure.
A Latinate rival was on the table. Middle English also used nativitee, from Old French nativité and Late Latin nativitas, for birth, descent, and the feast of Christ’s nativity. That line stayed strong in church and learned talk. For the ordinary dating of a person’s year, the native compound kept the field.
Bearing, process, and daylight
Birth is a process-noun on a very old verb. Behind Old Norse burðr and Old English ġebyrd sit Proto-Germanic *burdiz and the prefixed *gaburþiz. Those go back to a past-participle formation of the root that gives English bear in the senses carry and give birth. The deeper root is Proto-Indo-European *bher-, the family of Latin ferre, to carry. The Germanic ending -th (as in death and bath) marks the process or its result. So the first half of birthday is, morphologically, the bearing itself, not a decorative prefix.
Day is native on the other side: Old English dæg, from Proto-Germanic *dages-, with close kin in Dutch dag, German Tag, Old Norse dagr, and Gothic dags. Its prehistory is less settled. One reconstruction ties it to a root *agh- meaning a day, then has to treat the Germanic initial d- as unclear. Another ties it to *dhegh-, to burn, which would make the word a daylight word at heart. It is not related to Latin dies, despite the shared meaning; that Latin noun comes from a root for shining. In Old English dæg named the hours when the sun was up, and also a span of life. The civil twenty-four-hour day is a later widening, already in late Anglo-Saxon use.
German Geburtstag, Dutch geboortedag, Low German Geboortsdag, Saterland Frisian Gebuursdai, and Norwegian bursdag or gebursdag stack the same two ideas in the same order. English looks simpler only because it lost the ge- that several of those cousins still wear.
Saints, sovereigns, then everyone
Early use did not assume that every household kept a personal feast. Accounts that follow the Old English compound describe it as an anniversary or celebration of a birth, at first typically a king’s or a saint’s. That fits a world in which the church calendar and royal ritual stored public memory of nativity, while most people might not record, or at least not feast, their own.
A second sense — the day on which a person was actually born, not only the yearly return of that date — is dated to the 1570s in one widely used etymology. Another source attaches that same 1570s dating to the anniversary sense instead. The references do not agree which meaning is the later one. They do agree that Early Modern English is when the noun is firmly on record for ordinary personal dating, not only for the great. By the 1620s English also had birthnight, the evening counterpart, which only makes sense once the occasion is a social event and not merely a liturgical commemoration.
The official birthday of a monarch still shows the older public use: the date the state observes need not be the natal day. That split — calendar fact versus ceremonial anniversary — is the same double job the noun has always been able to do.
Later extensions follow the path of birth itself. Once birth could mean any coming into existence, a building, a magazine, or a republic could have a birthday without a midwife. Informal English even made a verb of the noun: to birthday, to keep the day. Lewis Carroll’s unbirthday, from 1874, works only because the positive form had become the default personal holiday, the day that needs no extra label.
Archaic spellings such as birth-day and birthdaye make the compound’s joint visible on the page. What changed was not the sense so much as the circle of people entitled to the word.
The remodeling that stuck
Keep one concrete fact from the history. English already named the day in Old English. Norse-influenced birth then replaced ġebyrd inside the compound, and the French-Latin nativity word did not win the annual party. German Geburtstag is the older morphology still wearing its prefix. English birthday is the same Germanic pair, sanded down until the join is obvious — and until a private anniversary, not only a saint’s or a king’s, can wear the name.