An ordinary exit word
In present-day English, goodbye is the default thing to say when a conversation, a visit, or a phone call is ending. It works as an interjection, as a noun for the leave-taking itself, and, more rarely, as a verb. Most speakers feel no theology in it. They hear a polite close, or a slightly heavier one than a clipped bye. The religious wish that once sat inside the form is no longer part of ordinary competence. That buried history is the origin of the word goodbye.
The modern range is wide but shallow. You can wave a goodbye, kiss someone goodbye, or, figuratively, say goodbye to a job or a habit. Those later uses grew once the interjection was common enough to name the event around it.
Built in English, not borrowed
Unlike many English greetings that arrived ready-made from French or Latin, this one was assembled at home. Lexicographers treat it as a native clipping of the blessing God be with you, and of the variant God be with ye. The unshortened formula is already on record as a parting wish in the late fifteenth century. An English printing from 1483 has a departing speaker use that full blessing. Further examples appear through the sixteenth century.
The squeezed shape belongs to the late 1500s. The earliest firm evidence for the contracted noun is from 1575, in a letter-book of the scholar Gabriel Harvey, who already treats godbwyes as countable farewells and jokes them against how-do-ye greetings. Nearby sixteenth-century spellings wander through forms such as God be wy you, God b'uy, God buoye, and God buy. The spelling that looks like today’s good-bye is in place by the 1590s.
This is inheritance by shortening, not a loan. The pieces are English: God from Old English god, itself from Proto-Germanic gudą; be from Old English bēon; with from Old English wiþ; you and ye from Old English second-person pronouns. Deeper Indo-European ancestry for the god-word is uncertain. What matters is that four ordinary English words were hurried into one routine.
Why God became good
The first syllable did not remain God. Speakers swapped in the adjective good. The usual explanation is analogical: English already had a family of time-of-day formulas built on that adjective, including good day, good morning, good evening, and good night. Once the blessing had been compressed, it was easy to hear it as another member of that family. Standard dictionaries describe the change as an alteration of God be with you under the pull of good day. After that swap, the word looks, to a modern eye, like good plus bye, even though bye was not originally a separate morpheme.
A minority account, published by W. Franz in 1898, derived the form from God buy you, with buy in an older sense close to redeem. That story would help explain the diphthong in the second syllable, which does not fall out automatically from unstressed be with. It is not supported by the earliest spellings, which still show the be-with structure. Later buy spellings do exist, and they may have colored how the word was pronounced even if they are not the starting point. The honest summary is that the main line of evidence points to God be with you or ye, with a secondary tangle involving buy.
The pronoun in the original phrase is also worth a pause. You and ye were not always interchangeable. In unstressed speech the object form you could reduce, and some late spellings may show ye as the object of with. Either way, the last syllable of goodbye is a worn-down personal pronoun, not a newly invented ending.
From blessing to all-purpose close
In early use the contracted form still behaved like a blessing aimed at the person remaining behind. Farewell, older in English and built on fare in the sense of travel or go, was the word aimed at the person setting out. That directional etiquette did not last. Over time goodbye became a general English valediction, used by whoever was leaving and whoever was staying. The sense of commending someone to God faded as the sounds fused and as everyday speech secularized the formula.
Spelling followed speech rather than the other way around. Early texts sometimes keep spaces between the old words and sometimes write a solid string from the start. Later centuries favored a hyphen, giving good-bye and the shorter good-by. The closed form goodbye is now the usual choice in many house styles, though hyphenated spellings survive, and informal bye, 'bye, and g'bye are simply further reductions of the same word. Analyzing the modern form as good plus bye is folk morphology: it matches how English speakers now parse it, not how the phrase was first built.
As a noun it first named an instance of saying the word, then any gesture of leave-taking: a wave, a smile, a last look at a doorway. Figurative uses followed in the same track. To kiss something goodbye is to accept that it is lost. Those extensions are semantic, not etymological.
Cousins in sense, not a Germanic twin
True cognates attach to the old pieces, not to the fused farewell. God matches German Gott and Dutch god. Good matches German gut and Dutch goed, a different Old English adjective that only later occupied the first syllable of the parting word. The compound itself has no medieval German twin of the same shape. Germanic languages more often send someone off with a wish to meet again, or with a command to fare well, rather than with this particular four-word crush.
What English goodbye does have are close semantic cousins formed the same way in Romance: French adieu, Spanish adiós, Italian addio, and Portuguese adeus, each built on a preposition plus the local word for God. Those words, like the English blessing before it was clipped, were originally a commendation, not a cheerful wave. English speakers have long noticed the parallel, which is a useful check on folk stories that treat goodbye as a transparent wish for a good departure. The goodness in the modern spelling is an accident of analogy, not the original wish.
The concrete detail worth keeping is small. When the second syllable of goodbye sounds like the verb buy, that may be a later echo, not the original verb. The first records still look like a hurried God be with ye. The adjective good is the costume the blessing put on so it could stand beside good night.