Old English broþor, from Proto-Germanic *brothar (cf. Old Norse broðir, Danish broder, Old Frisian brother, Dutch broeder, German Bruder, Gothic bróþar), from PIE root *bhrater (cf. 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 "brother").
A highly stable word across the Indo-European languages. 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 where there was need to distinguish "son of the same mother" and "son of the same father." E.g. Greek adelphos, probably originally an adjective with frater and meaning, specifically, "brother of the womb" or "brother by blood;" and Spanish hermano "brother," from Latin germanus "full brother." 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.
(1.) In the natural and common sense (Matt. 1:2; Luke 3:1, 19). (2.) A near relation, a cousin (Gen. 13:8; 14:16; Matt. 12:46; John 7:3; Acts 1:14; Gal. 1:19). (3.) Simply a fellow-countryman (Matt. 5:47; Acts 3:22; Heb. 7:5). (4.) A disciple or follower (Matt. 25:40; Heb. 2:11, 12). (5.) One of the same faith (Amos 1:9; Acts 9:30; 11:29; 1 Cor. 5:11); whence the early disciples of our Lord were known to each other as brethren. (6.) A colleague in office (Ezra 3:2; 1 Cor. 1:1; 2 Cor. 1:1). (7.) A fellow-man (Gen. 9:5; 19:7; Matt. 5:22, 23, 24; 7:5; Heb. 2:17). (8.) One beloved or closely united with another in affection (2 Sam. 1:26; Acts 6:3; 1 Thess. 5:1). Brethren of Jesus (Matt. 1:25; 12:46, 50: Mark 3:31, 32; Gal. 1:19; 1 Cor. 9:5, etc.) were probably the younger children of Joseph and Mary. Some have supposed that they may have been the children of Joseph by a former marriage, and others that they were the children of Mary, the Virgin's sister, and wife of Cleophas. The first interpretation, however, is the most natural.