Online Etymology Dictionary

B movie (n.): by 1939, usually said to be so called from being the second, or supporting, film in a double feature. Some film industry sources say it was so called for being the second of the two films major studios generally made in a year, and the one made with less headline talent and released with less promotion. And early usage varies with grade-B movie, suggesting a perceived association with quality.
b'hoy (n.): 1846, U.S. colloquial for "spirited lad, young spark," supposedly from the Irish pronunciation of boy.
B'nai B'rith (n.): Jewish fraternal organization founded in New York City in 1843, Hebrew, literally "Sons of the Covenant," from bene, state construct of banim, plural of ben "son," + brith "covenant."
B-girl (n.): 1936, abbreviation of bar girl, U.S. slang for a woman paid to encourage customers at a bar to buy her drinks.
B.B.C.: acronym for British Broadcasting Corporation, established 1927 replacing British Broadcasting Company, so the acronym itself dates to 1923. BBC English as a type of standardized English recommended for announcers is recorded from 1928.
B.C.: abbreviation of Before Christ, in chronology, attested by 1823. The phrase itself, Before Christ, in dating, with exact years, is in use by 1660s.
B.C.E.: initialism for "Before Common Era" or "Before Christian Era," 1881; see C.E. A secular alternative to B.C.
b.o.: abbreviation of body odor, by c.1950; an advertisers' invention.
baa: imitative sound of a sheep, attested from 1580s, but probably older, as baa is recorded before this a name for a child's toy sheep.
Baal: "The name of many deities of the Semitic peoples" [Klein], late 14c., Biblical use is from Hebrew Ba'al, literally "owner, master, lord," from ba'al "he took possession of," also "he married;" related to Akkadian Belu (source of Hebrew Bel), name of Marduk. Also related to the first element in Beelzebub. Used figuratively for any "false god."
Baath: pan-Arab socialist party, founded by intellectuals in Syria in 1943, from Arabic ba't "resurrection, renaissance."
baba (n.): kind of plum cake, 1827, from French baba (19c.), said by French etymology dictionaries to be from Polish baba.
Babbitt (n.): "conventional, complacent, materialistic American businessman," 1923, from George Babbitt, title character of Sinclair Lewis' novel (1922).
His name was George F. Babbitt. He was forty-six years old now, in April 1920, and he made nothing in particular, neither butter nor shoes nor poetry, but he was nimble in the selling of houses for more money than people could afford to pay. [Sinclair Lewis, "Babbitt," 1922]
babble (v.): mid-13c., babeln "to prattle, chatter," akin to other Western European words for stammering and prattling (cf. Swedish babbla, Old French babillier) attested from the same era, some of which probably were borrowed from others, but etymologists cannot now determine which were original. Probably imitative of baby-talk, in any case (cf. Latin babulus "babbler," Greek barbaros "non-Greek-speaking"). "No direct connexion with Babel can be traced; though association with that may have affected the senses" [OED]. Meaning "to repeat oneself incoherently, speak foolishly" is attested from c.1400. Related: Babbled; babbling.
babble (n.): "idle talk," c.1500, from babble (v.). In 16c., commonly in reduplicated form bibble-babble.
babe (n.): late 14c., short for baban (early 13c.), which probably is imitative of baby talk (cf. babble), however in many languages the cognate word means "old woman" (cf. Russian babushka "grandmother," from baba "peasant woman").
Crist crid in cradil, "moder, baba!" [John Audelay, c.1426]
Now mostly superseded by its diminutive form baby. Used figuratively for "a childish person" from 1520s. Meaning "attractive young woman" is 1915, college slang. Babe in the woods is from 1795.
Babel: capital of Babylon, late 14c., from Hebrew Babhel (Gen. ix), from Akkadian bab-ilu "Gate of God" (from bab "gate" + ilu "god"). The name is a translation of Sumerian Ka-dingir. Meaning "confused medley of sounds" (1520s) is from the biblical story of the Tower of Babel.
babelicious (adj.): 1991, from babe in the "attractive young woman" sense + ending from delicious.
Babism (n.): 1850; see Baha'i.
baboon (n.): type of ape, c.1400, babewyn, earlier "a grotesque figure used in architecture or decoration" (early 14c.), from French babouin "baboon," from Old French baboin "ape," earlier "simpleton, dimwit, fool" (13c.), also "gaping figure (such as a gargoyle)," so perhaps from Old French baboue "grimacing;" or perhaps it is imitative of the ape's babbling speech-like cries. Also cf. -oon. German Pavian "baboon" is from Dutch baviaan, from Middle Dutch baubijn, a borrowing of the Old French word.
babouche (n.): 1690s, from French babouche, from Arabic babush, from Persian papush, from pa "foot" (related to Avestan pad-, see foot (n.)) + posh "covering." Arabic, lacking a -p-, regularly converts -p- in foreign words to -b-.
babushka (n.): type of head covering for women, 1938, from Russian babushka "grandmother."
baby (n.): late 14c., babi, diminutive of baban (see babe + -y (3)). Meaning "childish adult person" is from c.1600. Meaning "youngest of a group" is from 1897. As a term of endearment for one's lover it is attested perhaps as early as 1839, certainly by 1901; its popularity perhaps boosted by baby vamp "a popular girl," student slang from c.1922. As an adjective, by 1750.

Baby food is from 1833. Baby blues for "blue eyes" recorded by 1892 (the phrase also was used for "postpartum depression" 1950s-60s). To empty the baby out with the bath (water) is first recorded 1909 in G.B. Shaw (cf. German das Kind mit dem Bade ausschütten). Baby's breath (noted for sweet smell, which also was supposed to attract cats) as a type of flower is from 1897. French bébé (19c.) is from English.
baby (v.): "to treat like a baby," 1742, from baby (n.). Related: Babied; babying.
baby boom (n.): coined 1941, from baby (n.) + boom (n.); derivative baby-boomer (member of the one that began 1945) recorded by 1974.
babyish (adj.): 1753, from baby (n.) + -ish. Earlier in same sense was babish (1530s).
Babylon: mid-14c., from Greek version of Akkadian Bab-ilani "the gate of the gods," from bab "gate" + ilani, plural of ilu "god" (cf. Babel). The Old Persian form, Babiru-, shows characteristic transformation of -l- to -r- in words assimilated from Semitic.
Babylonian (n.): 1560s; see Babylon + -ian. From 1630s as an adjective. Earlier in the adjectival sense was Babylonical (1530s).
babysit (v.): also baby-sit, 1947, from baby (n.) + sit (v.); figurative use (often contemptuous) by 1968. Babysitting is from 1946.
babysitter (n.): also baby-sitter, 1914, from baby (n.) + agent noun from sit (v.). Short form sitter is attested from 1943.
Bacardi: 1921, name for a brand of West Indian rum produced by Compania Ron Bacardi, originally of Cuba.
baccalaureate (n.): 1620s, "university degree of a bachelor," from Medieval Latin baccalaureatus, from baccalaureus "student with the first degree," altered by a play on words with bacca lauri "laurel berry" (laurels being awarded for academic success).

The Medieval Latin word perhaps ultimately is derived from Latin baculum "staff" (see bacillus), which the young student might carry, but it is more likely just a re-Latinization of bachelor (q.v.) in its academic sense. In modern U.S. usage, the word usually is short for baccalaureate-sermon (1864), a religious farewell address to the graduating class.
baccarat (n.): card game, 1848, from French baccara (19c.), of unknown origin. Baccarat is the name of a town in France that was noted for glass-making.
Bacchae (n.): "female attendants of Bacchus," from Greek Bakkhai, plural of Bakkhe, from Bakkhos (see Bacchus).
bacchanal: 1530s (n.); 1540s (adj.), from Latin bacchanalis "having to do with Bacchus" (see Bacchus). Meaning "riotous, drunken roistering; orgy" is from 1711.
bacchanalia (n.): "drunken revelry," 1630s, from the name of the Roman festival held in honor of Bacchus, from neuter plural of Latin bacchanalis (see bacchanal). A participant is a Bacchant (1690s), fem. Bacchante, from French. The plural of both is Bacchantes.
bacchanalian (adj.): 1560s; see bacchanalia + -an. As a noun from 1610s.
Bacchus: Greek god of wine and revelry, late 15c., from Latin Bacchus, from Greek Bakkhos, perhaps related to Latin bacca "berry, olive-berry, bead, pearl." Perhaps originally a Thracian fertility god.
bach (n.): 1845, American English, clipped form of bachelor (n.). Also in colloquial American English use as a verb (1870) meaning "to live as an unmarried man," especially "to do one's own cooking and cleaning." Related: Bached; baching.
bachelor (n.): c.1300, "young man;" also "youthful knight, novice in arms," from Old French bacheler (11c.) "knight bachelor," a young squire in training for knighthood, of uncertain origin, perhaps from Medieval Latin baccalarius "vassal farmer," one who helps or tends a baccalaria "section of land." Or from Latin baculum "a stick," because the squire would practice with a staff, not a sword. Meaning evolved from "knight in training" to "young unmarried man" (early 14c.). Bachelor party as a pre-wedding ritual is from 1882.
bachelorette (n.): 1935, American English, from bachelor with French ending -ette. Replaced earlier bachelor-girl (1895). Middle French had bachelette "young girl;" Modern French bachelière is found only in the "student" sense.
bacilli (n.): plural of bacillus (q.v.).
bacillus (n.): 1877, medical Latin, from Late Latin bacillus "wand," literally "little staff," diminutive of baculum "a stick," from PIE root *bak- "staff," also source of Greek bakterion (see bacteria). Introduced as a term in bacteriology 1853 by German botanist Ferdinand Cohn (1828-1898).
back (n.): Old English bæc "back," from Proto-Germanic *bakam (cf. Old Saxon and Middle Dutch bak, Old Frisian bek), with no known connections outside Germanic.

The cognates mostly have been ousted in this sense in other modern Germanic languages by words akin to Modern English ridge (cf. Danish ryg, German Rücken). Many Indo-European languages show signs of once having distinguished the horizontal back of an animal (or a mountain range) from the upright back of a human. In other cases, a modern word for "back" may come from a word related to "spine" (Italian schiena, Russian spina) or "shoulder, shoulder blade" (Spanish espalda, Polish plecy).

To turn (one's) back on (someone or something) "ignore" is from early 14c. Behind (someone's) back "clandestinely" is from late 14c.

To know (something) like the back of one's hand, implying familiarity, is first attested 1893. The first attested use of the phrase is from a dismissive speech made to a character in Robert Louis Stevenson's "Catriona":
If I durst speak to herself, you may be certain I would never dream of trusting it to you; because I know you like the back of my hand, and all your blustering talk is that much wind to me.
The story, a sequel to "Kidnapped," has a Scottish setting and context, and the back of my hand to you was noted in the late 19th century as a Scottish expression meaning "I will have nothing to do with you" [e.g. "Jamieson's Dictionary of the Scottish Language"]. In English generally, the back of (one's) hand has been used to imply contempt and rejection since at least 1300. Perhaps the connection of a menacing dismissal is what made Stevenson choose that particular anatomical reference.
back (v.): late 15c., "to move (something) back," from back (adv.). Meaning "to support" (as by a bet) is first attested 1540s. Related: Backed; backing.
back (adj.): Middle English, from back (n.) and back (adv.). Formerly with comparative backer (c.1400), also backermore. To be on the back burner in the figurative sense is from 1960, from the image of a cook keeping a pot there to simmer while he or she works on another concoction at the front of the stove.
back (adv.): late 14c., shortened from abak, from Old English on bæc "backwards, behind, aback" (see back (n.)). Back and forth attested from 1814.
back down (v.): in figurative sense of "withdraw a charge," 1859, American English, from notion of descending a ladder, etc.; from back (v.) + down (adv.).
back off (v.): "retreat, stop annoying someone," by 1938, from back (v.) + off.
back seat (n.): also back-seat, 1832, originally of coaches, from back (adj.) + seat (n.). Back-seat driver first attested 1926.