best (adj.) Look up best at Dictionary.com
Old English beste, reduced by assimilation of -t- from earlier Old English betst "best, first, in the best manner," originally superlative of bot "remedy, reparation," the root word now only surviving in to boot (see boot (n.2)), though its comparative, better, and superlative, best, have been transferred to good (and in some cases well). From Proto-Germanic root *bat-, with comparative *batizon and superlative *batistaz (cognates: Old Frisian, Old Saxon, Middle Dutch best, Old High German bezzist, German best, Old Norse beztr, Gothic batists).
The best-laid schemes o' mice an' men
Gang aft agley,
An' lea'e us nought but grief an' pain,
For promis'd joy!

[Burns]
Best-seller as short for "best-selling book" is from 1902, apparently originally in the publishing trade; best friend was in Chaucer (late 14c.). Best girl is first attested 1881, American English; best man is 1814, originally Scottish, replacing groomsman. To be able to do something with the best of them is recorded by 1748.
best (v.) Look up best at Dictionary.com
"to get the better of," 1863, from best (adj.). Related: Bested; besting.
best (n.) Look up best at Dictionary.com
c.1200, from best (adj.).
bestead (v.) Look up bestead at Dictionary.com
"to help, support, prop," 1580s, from be- + stead (v.); see stead.
bestest (adj.) Look up bestest at Dictionary.com
jocular emphatic superlative of best (which is itself a superlative), attested from 1834.
bestial (adj.) Look up bestial at Dictionary.com
late 14c., from Old French bestial (13c.) "relating to animals, stupid, foolish, bestial" and directly from Latin bestialis "like a beast," from bestia (see beast). Sense of "below the dignity of a human" is from c.1400, and in many cases its use is unjust to the beasts.
bestiality (n.) Look up bestiality at Dictionary.com
late 14c., "the nature of beasts," from bestial + -ity. Meaning "indulgence in beastly instincts" is from 1650s; sense of "sexual activity with a beast" is from 1611 (KJV).
bestiary (n.) Look up bestiary at Dictionary.com
"medieval treatise on beasts" usually with moralistic overtones, 1818, from Medieval Latin bestiarium "a menagerie," also "a book about animals", from bestia (see beast). A Latin term for such works was liber de bestiis compositus. Roman bestiarius meant "a fighter against beasts in the public entertainments."
bestir (v.) Look up bestir at Dictionary.com
Old English bestyrian "to heap up," from be- + stir. Related: Bestirred; bestirring.
bestow (v.) Look up bestow at Dictionary.com
early 14c., bistowen "give" (as alms, etc.), from be- + stowen "to place" (see stow). Related: Bestowed; bestowing; bestower.
bestowal (n.) Look up bestowal at Dictionary.com
1773, from bestow + -al (2).
bestrew (v.) Look up bestrew at Dictionary.com
Old English bestreowian; see be- + strew (v.).
bestride (v.) Look up bestride at Dictionary.com
Old English bestridan "to bestride, mount," from be- + stridan "to stride" (see stride). Compare Middle Dutch bestryden.
bet Look up bet at Dictionary.com
1590s, as both a verb and noun, in the argot of petty criminals, of unknown origin; probably a shortening of abet or else from obsolete beet "to make good," from Old English bætan "make better, arouse, stimulate," from Proto-Germanic *baitjan, in which case the verb would be the original. The original notion is perhaps "to improve" a contest by wagering on it, or it is from the "bait" sense in abet. Used since 1852 in various American English slang assertions (compare you bet "be assured," 1857). Related: Betting.
beta (n.) Look up beta at Dictionary.com
second letter of the Greek alphabet, c.1300, from Greek, from Hebrew/Phoenician beth (see alphabet); used to designate the second of many things. Beta radiation is from 1899 (Rutherford). Beta particle is attested from 1904.
betake (v.) Look up betake at Dictionary.com
c.1200, from be- + take. Related: Betook; betaken.
Betamax (n.) Look up Betamax at Dictionary.com
1975, proprietary name (Sony), from Japanese beta-beta "all over" + max, from English maximum.
betcha Look up betcha at Dictionary.com
representing casual pronunciation of bet you, attested by 1904.
bete noire (n.) Look up bete noire at Dictionary.com
"insufferable person," 1844, from French bête noire "personal aversion," as an adjective, "stupid, foolish;" literally "the black beast."
beteach (v.) Look up beteach at Dictionary.com
Old English betæcan, from be- + teach. Related: Betaught; beteaching.
betel (n.) Look up betel at Dictionary.com
1550s, probably via Portuguese betel, from Malayalam vettila, from veru ila "simple leaf."
Betelgeuse Look up Betelgeuse at Dictionary.com
bright star in the shoulder of Orion, 1515, from Arabic Ibt al Jauzah "the Armpit of the Central One." Intermediary forms include Bed Elgueze, Beit Algueze.
Bethany Look up Bethany at Dictionary.com
Biblical village, its name in Hebrew or Aramaic is literally "house of poverty," from bet "house of" (construct state of bayit "house") + 'anya "poverty."
bethel (n.) Look up bethel at Dictionary.com
1610s, "a place where God is worshipped," from Hebrew beth El "house of God," from beth, construct state of bayit "house." Popular as a name for religious meeting houses among some Protestant denominations. Beth also was the name of the second letter of the Hebrew alphabet, so called for its shape, and was borrowed into Greek as beta.
Bethesda Look up Bethesda at Dictionary.com
1857, name of a pool in Jerusalem (John v:2), from Greek Bethesda, from Aramaic beth hesda "house of mercy," or perhaps "place of flowing water." Popular as a name for religious meeting houses among some Protestant denominations.
bethink (v.) Look up bethink at Dictionary.com
reflexive verb, Old English beþencan "to consider," from be- + þencan "to think" (see think). Related: Bethought.
Bethlehem Look up Bethlehem at Dictionary.com
the name probably means "House of Lahmu and Lahamu," a pair of Mesopotamian agricultural deities.
betide (v.) Look up betide at Dictionary.com
"to happen, befall," late 12c., from be- + tiden "to happen" (see tide).
betimes (adv.) Look up betimes at Dictionary.com
"at an early period," early 14c., from betime (c.1300, from be- + time) + adverbial genitive -s.
betoken (v.) Look up betoken at Dictionary.com
late 12c., from be- + Old English tacnian "to signify," from tacn "sign" (see token). Related: Betokened; betokening.
betray (v.) Look up betray at Dictionary.com
late 13c., bitrayen "mislead, deceive, betray," from be- + obsolete Middle English tray, from Old French traine "betrayal, deception, deceit," from trair (Modern French trahir) "betray, deceive," from Latin tradere "hand over," from trans- "across" (see trans-) + dare "to give" (see date (n.1)). Related: Betrayed; betraying.
betrayal (n.) Look up betrayal at Dictionary.com
1816; from betray + -al (2). Earlier in the same sense were betrayment (1540s), betraying (late 14c.).
betrayer (n.) Look up betrayer at Dictionary.com
1520s, agent noun from betray (v.).
betroth (v.) Look up betroth at Dictionary.com
c.1300, betrouthen, from bi-, here probably with a sense of "thoroughly," + Middle English treowðe "truth," from Old English treowðe "truth, a pledge" (see troth). Related: Betrothed; betrothing.
betrothal (n.) Look up betrothal at Dictionary.com
1844, from betroth + -al (2). Earlier in same sense were betrothment (1580s), betrothing (14c.).
betrothed (adj.) Look up betrothed at Dictionary.com
1530s, past participle adjective from betroth (v.). As a noun, in use by 1580s.
Betsy Look up Betsy at Dictionary.com
fem. pet name, a diminutive of Bet, itself short for Elizabet or Elizabeth. Betsy as the typical a pet name for a favorite firearm is attested in American English by 1856 (compare Brown Bess, by 1785, British army slang for the old flintlock musket).
better (adj.) Look up better at Dictionary.com
Old English bettra, earlier betera, from Proto-Germanic *batizo-, from PIE *bhad- "good;" see best. Comparative adjective of good in the older Germanic languages (compare Old Frisian betera, Old Saxon betiro, Old Norse betr, Danish bedre, Old High German bezziro, German besser, Gothic batiza). In English it superseded bet in the adverbial sense by 1600. Better half "wife" is first attested 1570s.
better (v.) Look up better at Dictionary.com
Old English *beterian "improve, amend, make better," from Proto-Germanic *batizojan (cognates: Old Frisian beteria, Dutch beteren, Old Norse betra, Old High German baziron, German bessern), from *batiz- (see better (adj.)). Related: Bettered; bettering.
better (n.) Look up better at Dictionary.com
late 12c., "that which is better," from better (adj.). Specific meaning "one's superior" is from early 14c. To get the better of (someone) is from 1650s, from better in a sense of "superiority, mastery," which is recorded from mid-15c.
betterment (n.) Look up betterment at Dictionary.com
1590s, from better (v.) + -ment.
bettor (n.) Look up bettor at Dictionary.com
also better (OED notes that English agent nouns in -er tend to shift toward -or as their senses become more specific), agent noun from bet (v.).
Betty Look up Betty at Dictionary.com
fem. pet name, from Bet, shortened from Elizabeth, + -y (3).
Betula (n.) Look up Betula at Dictionary.com
genus of the birches, from Latin betula "birch," from Gaulish betu- "bitumen" (cognates: Middle Irish beithe "box tree," Welsh bedwen "birch tree"). According to Pliny, so called because the Gauls extracted tar from birches. Birch tar is still sold as an analgesic and stimulant and made into birch beer by the Pennsylvania Dutch.
between (prep.) Look up between at Dictionary.com
Old English betweonum "between, among, by turns," Mercian betwinum, from bi- "by" (see be-) + tweonum dative plural of *tweon "two each" (compare Gothic tweih-nai "two each"). Between a rock and a hard place is from 1940s, originally cowboy slang. Between-whiles is from 1670s.
betweenity (n.) Look up betweenity at Dictionary.com
1760, a jocular formation, perhaps coined by Horace Walpole, from between + -ity.
betweenness (n.) Look up betweenness at Dictionary.com
1881, from between + -ness.
betwixt (prep., adv.) Look up betwixt at Dictionary.com
Old English betweox "between, among, amidst, meanwhile," from bi- "by" (see be-) + tweox "for two," from Proto-Germanic *twa "two" + *-isk "-ish." With parasitic -t that first appeared in Old English and became general after c.1500.
Beulah Look up Beulah at Dictionary.com
fem. proper name, from Hebrew be'ulah "married woman," fem. past participle of ba'al "he married" (see baal).
bevel (adj.) Look up bevel at Dictionary.com
1560s, possibly from Old French *baivel (Modern French béveau, biveau), possibly from bayer "to gape, yawn," from Latin *batare "to yawn, gape," from Latin root *bat-, possibly imitative of yawning. If so, the time gap is puzzling. The verb is first recorded 1670s. The noun is 1670s, from the adjective.