A word for kinship and company
In everyday English, brother names a male sibling: a son of the same parents, or of one shared parent. The same form also stretches beyond the household. Speakers use it for a fellow member of a church, a union, a lodge, or a religious house; as a familiar way of addressing another man; and, in African American English, for a fellow Black man. Those extra uses follow a long pattern in which the word for a male sibling has also named allies and men of the same group.
The origin of the word brother is not a borrowing from French or Latin, though English later imported cousins of the same root through those languages. It is a native Germanic inheritance, already in Old English as brōþor (also written broþor), and recorded before the twelfth century, in texts that go back well before the year 1000.
From Old English back through Germanic
Old English brōþor came down from Proto-West Germanic *brōþer and, behind that, Proto-Germanic *brōþēr. The same ancestor produced a tight cluster of Germanic siblings: Old Norse broðir, Gothic brōþar, Old High German bruodar, German Bruder, Dutch broeder, Old Frisian brother. English kept its own line from that prehistoric stock; it did not borrow the word from any of those later languages.
The consonant in the middle of the Germanic forms is the sound written þ. That sound is the regular Germanic outcome of an older t, by the shift known as Grimm’s Law. The related Latin word is frater, with t still in place; English brother has the shifted consonant. The same law turned the initial voiced aspirate of the older root into Germanic b, so the word begins with b in English, German, and Gothic, while Latin shows f, the Italic treatment of the same older beginning.
A textbook contrast is the word for father, reconstructed as *ph₂tḗr. It became Proto-Germanic *fadēr, with a voiced consonant, not the *faþēr that Grimm’s Law alone would suggest. That voicing is Verner’s Law, which applied when the older accent did not fall on the immediately preceding syllable. Brother had its accent on the first syllable, so the t shifted to þ and stayed there. English still carries the difference: father with a d-sound, brother with th.
Middle English wrote brother, broþer, and also forms with d, such as broder. Those d-spellings reflect later sound change in some dialects and analogy with father and mother. The modern spelling settled on th, matching the inherited Germanic consonant.
The Indo-European kinship word
Behind Proto-Germanic *brōþēr lies Proto-Indo-European *bʰréh₂tēr. The reconstruction is widely accepted. The word belongs to a small set of old kinship nouns built with a suffix of the shape *-tēr, the same family of endings seen in the words for father and daughter. Sanskrit preserves bhrātár-; Latin has frāter; Old Irish bráthair; Welsh brawd; Russian brat; Lithuanian brólis. The match is one of the most stable equations in the Indo-European lexicon.
Greek is the famous wrinkle. Ancient Greek phrātēr did not mainly mean a male sibling. It named a member of a phratry, a clan or civic brotherhood. The ordinary word for a blood brother became adelphos, originally a more specific companion of phrātēr, something like a brother of the same womb. Some linguists therefore suspect that the Proto-Indo-European word already covered kinsman or fellow of the group as well as sibling. Celtic and Baltic also show broader senses in places. Others treat those wider meanings as later, independent drifts. The evidence does not force a single answer; what is clear is that the root was available for both tight family and looser fellowship.
English later borrowed the Latin line separately. Friar comes from Old French frere, from Latin frater. Fraternal, fraternity, and fratricide are learned Latin formations. They are doublets of the native word, not replacements of it. When a Romance language needed a clear term for a blood sibling after the old cognate had drifted toward monastic or fraternal use, it sometimes chose another Latin word: Spanish hermano comes from germanus, a full brother on both sides. Middle English even had the compound brother-german for that stricter sense.
Albanian does not keep the old root, and a proposed Anatolian cognate is doubtful, but those gaps do not weaken the reconstruction where the word survives.
Senses that opened out
The core sibling meaning is continuous from Old English. Around the late fourteenth century the word is also used for a male of the same ancestry more broadly, not only a child of the same parents. By about 1500 it names a member of a mendicant religious order, the same social slot occupied by friar on the French side. Address from one man to another as brother is recorded in United States slang from 1912; a more specific use among Black Americans is dated to 1973 in standard etymological notes, though the social history of the address is older and wider than a single citation year can show.
The plural history is as informative as the senses. Old English could use a plural identical to the singular, and also forms with a fronted vowel, related to the dative singular brēþer. Whether those front-vowel plurals are a native analogical extension from the dative, or whether Norse influence helped them spread in Middle English, is still argued. What is not in doubt is the next step: Middle English added the weak plural ending -en to those already distinct plurals, giving brethren, a double plural, like children. From roughly 1200 to the 1600s brethren was the ordinary plural. After the language generalized -s, brothers took over for sons of the same parents. Brethren remained for members of a church, a sect, or a brotherhood. The split in the plural maps the split in meaning: household on one side, fellowship on the other.
What the consonants still show
If you set English brother beside Latin frater and German Bruder, you are looking at one inherited noun, not three separate inventions. The English th is the Germanic trace of an older t; the Latin f and English b are two treatments of the same initial; and the surviving contrast with father records where the prehistoric accent once sat. The word did not need to be borrowed to name a fellow in a guild or a monastery. That extension was already latent in a kinship term that has long named both the boy in the same household and the man counted as one of us.