Origins
The English word 'brother' traces an unbroken line of descent from Proto-Indo-European *bʰréh₂tēr through Proto-Germanic *brōþēr to Old English 'brōþor' and onward to its modern form. It is one of the foundational kinship terms of the Indo-European language family, attested in virtually every branch with striking phonological regularity.
The PIE form *bʰréh₂tēr is reconstructed from an exceptionally wide set of cognates. Latin 'frāter' (source of French 'frère,' Spanish 'hermano' — though the latter replaced 'frāter' with 'germānus,' meaning 'genuine sibling'), Greek 'phrātēr,' Sanskrit 'bhrā́tar-,' Avestan 'brātar-,' Old Irish 'bráthir,' Old Church Slavonic 'bratrŭ' (source of Russian 'brat'), Lithuanian 'broterelis' (diminutive), Armenian 'ełbayr,' and Tocharian B 'procer' all descend from this single proto-form.
The initial consonant cluster *bʰr- is a diagnostic feature for tracking regular sound changes across branches. In Germanic, PIE *bʰ became *b by Grimm's Law (which deaspirated voiced aspirates), giving Proto-Germanic *br-. In Latin, *bʰ became f- by a parallel but independent process, yielding 'frāter.' In Greek, *bʰ became ph- (φ), producing 'phrātēr.' In Sanskrit, the aspirate was preserved directly as 'bh-.' These regular correspondences — b:f:ph:bh — were among the patterns that August Schleicher and the Neogrammarians used in the nineteenth century to establish the sound laws governing Indo-European daughter languages.
Proto-Indo-European Roots
The semantic history of *bʰréh₂tēr reveals something important about PIE social structure. While the word clearly meant 'male sibling' in most branches, the Greek reflex 'phrātēr' did not refer to a biological brother at all. Instead, it designated a 'fellow member of a phratry,' a kinship-based subdivision of the Greek tribe. The word for biological brother in Greek was 'adelphós' (literally 'from the same womb,' from 'a-' same + 'delphýs' womb). This divergence suggests that the PIE original may have carried a broader meaning encompassing male kinship solidarity — fellow clansmen who shared obligations of mutual support — with biological brotherhood being one specific instance of this wider bond.
The phonological development to Modern English is regular. Proto-Germanic *brōþēr had a long *ō vowel that persisted into Old English 'brōþor.' During the Middle English period, this long vowel shortened before the dental cluster, and the medial fricative /θ/ voiced to /ð/, giving the modern pronunciation with /ʌ/ (after the vowel unrounded) and /ð/. The Old English plural was 'brōþru' or 'brēþer,' and this latter form survives in the archaic plural 'brethren,' now used mainly in religious or fraternal contexts ('the brethren of the order'). The regular modern plural 'brothers' replaced the old form in everyday speech by the sixteenth century.
The Latin cognate 'frāter' entered English indirectly through extensive borrowing. 'Fraternal' (brotherly), 'fraternity' (brotherhood, especially in the institutional sense), 'fraternize' (to associate as brothers), and 'fratricide' (killing of a brother) all derive from 'frāter.' The word 'friar' — a member of a mendicant religious order — comes from Old French 'frere,' itself from Latin 'frāter,' reflecting the custom of addressing fellow monks as 'brother.' So English possesses both the Germanic 'brother' and the Latin 'friar,' two words from the same PIE root that entered the language by completely different routes and now occupy entirely different semantic spaces.
Modern Usage
Culturally, 'brother' has been one of the most productive metaphorical terms in English. 'Brotherhood' extends from biological kinship to trade unions ('brotherhood of railroad workers'), religious orders, and abstract ideals ('the brotherhood of man'). The African American use of 'brother' (and its shortened form 'bro') as a term of solidarity and community has become one of the most recognizable uses of the word in modern English, extending a metaphorical tradition that may be as old as the PIE word itself.
The compound 'brother-in-law' (attested from the fifteenth century) fills a gap that PIE handled differently. Proto-Indo-European had a specific term for 'husband's brother,' *deh₂iwḗr (source of Latin 'lēvir,' Greek 'dāḗr,' Sanskrit 'devár-'), but this word was lost in Germanic. English compensated with the compound 'brother-in-law,' a formation that treats the legal relationship as a modification of the core kinship term rather than as a separate concept entirely.