bhrāter-, Proto-Indo-European root meaning "brother." 

It forms all or part of: br'er; brethren; ‌‌brother; bully (n.); confrere; fraternal; fraternity; fraternize; fratricide; friar; friary; pal.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhrátár-, Old Persian brata, Greek phratér, Latin frater, Old Irish brathir, Welsh brawd, Lithuanian broterėlis, Old Prussian brati, Old Church Slavonic bratru, Czech bratr, Polish brat, Russian bratŭ, Kurdish bera; Old English broþor, Old Norse broðir, German Bruder, Gothic bróþar. 

also brer, in Br'er Rabbit, etc., 1881, Joel Chandler Harris' representation of U.S. Southern black pronunciation of brother.

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 adopted by primitive Christians (Acts xviii, etc.) and so was taken by various Protestant sects, such as the Dunkers.

colloquial abbreviation of brother, attested from 1660s.

14c., "fraternal relation, relationship between sons of the same father or mother," from brother + -hood; earlier was brotherhede (c. 1300), with ending as in maidenhead; and Old English had broþerrede, with ending as in kindred. The modern form of the word prevailed from 15c.

Originally "relationship of a brother," also "friendly companionship." The concrete sense of "an association of men for any purpose, a fraternity" is from mid-14c. in the Middle English word (later also "labor union," 1880s). The meaning "a class of individuals of the same kind" is from 1728. The meaning "community feeling uniting all humankind" is from 1784. Old English also had broðorscipe "brothership," broðorsibb "kinship of brothers."

What edge of subtlety canst thou suppose

Keen enough, wise and skilful as thou art,

To cut the link of brotherhood, by which

One common Maker bound me to the kind?

[Cowper, from "The Task," 1785]

***

Oh, the Protestants hate the Catholics,

And the Catholics hate the Protestants,

And the Hindus hate the Muslims,

And everybody hates the Jews.

[Tom Lehrer, "National Brotherhood Week" lyrics, 1965]

"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."

Middle English brotherli, "between brothers," from Old English broðorlic "of or pertaining to a brother;" see brother + -ly (1). The meaning "fraternal, kind, affectionate" is from 1530s.

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

familiar form of address for a male, 1851, perhaps a shortening of buddy (q.v.) and ultimately from brother.

1850, American English, possibly an alteration of brother, or from British colloquial butty "companion" (1802), itself perhaps a variant of booty in booty fellow "confederate who shares plunder" (1520s). But butty, meaning "work-mate," also was a localized dialect word in England and Wales, attested since 18c., and long associated with coal miners. Short form bud is attested from 1851. Reduplicated form buddy-buddy (adj.) attested by 1952, American English.

Lenny Kent, a long-time fave here, is really in his element. ... After four weeks here he's got everyone in town saying, "Hiya, Buddy, Buddy" with a drawl simulating his. [Review of Ned Schuyler's 5 O'Clock Club, Miami Beach, Fla., Billboard, Nov. 12, 1949]

Buddy system is attested from 1920.