Etymology
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brother (n.)

Old English broþor, from Proto-Germanic *brothar (source also of Old Norse broðir, Danish broder, Old Frisian brother, Dutch broeder, Old High German bruodar, German Bruder, Gothic bróþar), from PIE root *bhrater-.

A stable word across the Indo-European languages (Sanskrit bhrátár-, Greek phratér, Latin frater, etc.). Hungarian barát is from Slavic; Turkish birader is from Persian.

In the few cases where other words provide the sense, it is where the cognate of brother had been applied widely to "member of a fraternity," or as an appellation of a monk (Italian fra, Portuguese frade, Old French frere), or where there was need to distinguish "son of the same mother" from "son of the same father." For example Greek adelphos, which probably originally was an adjective with phrater and meant, specifically, "brother of the womb" or "brother by blood," and became the main word as phrater became "one of the same tribe." Spanish hermano "brother" is from Latin germanus "full brother" (on both the father's and mother's side); Middle English also had brother-german in this sense.

Meaning "male person in relation to any other person of the same ancestry" in English is from late 14c. Sense of "member of a mendicant order" is from c. 1500. As a familiar term of address from one man to another, it is attested from 1912 in U.S. slang; the specific use among blacks is recorded from 1973.

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brother-in-law (n.)
"brother of one's husband or wife," also "brother of one's sister's husband," c. 1300; also brother in law; see brother + in-law. In Arabic, Urdu, Swahili, etc., brother-in-law, when addressed to a male who is not a brother-in-law, is an extreme insult, with implications of "I slept with your sister."
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bro (n.)
colloquial abbreviation of brother, attested from 1660s.
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stepbrother (n.)
also step-brother, mid-15c., from step- + brother (n.).
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brotherly (adj.)
Old English broðorlic "of or pertaining to a brother;" see brother + -ly (1). Meaning "fraternal, kind, affectionate" is from 1530s.
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br'er 
also brer, in Br'er Rabbit, etc., 1881, Joel Chandler Harris' representation of U.S. Southern black pronunciation of brother.
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bud 
familiar form of address for a male, 1851, perhaps a shortening of buddy (q.v.) and ultimately from brother.
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bubba (n.)

familiar address or nickname for a male, 1873 (in a South Carolina context), Southern U.S. slang, said to be a corruption of brother.

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brethren (n.)
alternative plural of brother (q.v.); predominant c. 1200-1600s, but surviving only in religious usage and not now used in reference to male children of the same parents. The title was used by primitive Christians (Acts xviii, etc.) and so was taken by various Protestant sects, such as the Dunkers.
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