brother (n.) Look up brother at Dictionary.com
Old English broþor, from Proto-Germanic *brothar (source also of Old Norse broðir, Danish broder, Old Frisian brother, Dutch broþor "brother or half-brother by blood relation," also "one of two or more men closely united without regard for personal kinship," 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-, Old Persian brata, Greek phratér, Latin frater, Old Irish brathir, Welsh brawd, Lithuanian broterelis, Old Prussian brati, Old Church Slavonic bratru, Czech bratr, Polish brat, Russian bratŭ, Kurdish bera. 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.