Online Etymology Dictionary

best (adj.): 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 (cf. 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.): "to get the better of," 1863, from best (adj.). Related: Bested; besting.
best (n.): c.1200, from best (adj.).
bestead (v.): "to help, support, prop," 1580s, from be- + stead (v.); see stead.
bestest (adj.): jocular emphatic superlative of best (which is itself a superlative), attested from 1834.
bestial (adj.): 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.): 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.): "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.): Old English bestyrian "to heap up," from be- + stir. Related: Bestirred; bestirring.
bestow (v.): early 14c., bistowen "give" (as alms, etc.), from be- + stowen "to place" (see stow). Related: Bestowed; bestowing; bestower.
bestowal (n.): 1773, from bestow + -al (2).
bestrew (v.): Old English bestreowian; see be- + strew (v.).
bestride (v.): Old English bestridan "to bestride, mount," from be- + stridan "to stride" (see stride). Cf. Middle Dutch bestryden.
bet: 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 *baitjanan, 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 (cf. you bet "be assured," 1857). Related: Betting.
beta (n.): 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.): c.1200, from be- + take. Related: Betook; betaken.
Betamax (n.): 1975, proprietary name (Sony), from Japanese beta-beta "all over" + max, from English maximum.
betcha: representing casual pronunciation of bet you, attested by 1904.
bete noire (n.): "insufferable person," 1844, from French bête noire "personal aversion," as an adjective, "stupid, foolish;" literally "the black beast."
beteach (v.): Old English betæcan, from be- + teach. Related: Betaught; beteaching.
betel (n.): 1550s, probably via Portuguese betel, from Malayalam vettila, from veru ila "simple leaf."
Betelgeuse: 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: 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.): 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: 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.): reflexive verb, Old English beþencan "to consider," from be- + þencan "to think" (see think). Related: Bethought.
Bethlehem: the name probably means "House of Lahmu and Lahamu," a pair of Mesopotamian agricultural deities.
betide (v.): "to happen, befall," late 12c., from be- + tiden "to happen" (see tide).
betimes (adv.): "at an early period," early 14c., from betime (c.1300, from be- + time) + adverbial genitive -s.
betoken (v.): late 12c., from be- + Old English tacnian "to signify," from tacn "sign" (see token). Related: Betokened; betokening.
betray (v.): 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.): 1816; from betray + -al (2). Earlier in the same sense were betrayment (1540s), betraying (late 14c.).
betrayer (n.): 1520s, agent noun from betray (v.).
betroth (v.): 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.): 1844, from betroth + -al (2). Earlier in same sense were betrothment (1580s), betrothing (14c.).
betrothed (adj.): 1530s, past participle adjective from betroth (v.). As a noun, in use by 1580s.
Betsy: 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 (cf. Brown Bess, by 1785, British army slang for the old flintlock musket).
better (adj.): Old English bettra, earlier betera, from Proto-Germanic *batizo-, from PIE *bhad- "good;" see best. Comparative adjective of good in the older Germanic languages (cf. 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.): Old English *beterian "improve, amend, make better," from Proto-Germanic *batizojan (cf. Old Frisian beteria, Dutch beteren, Old Norse betra, Old High German baziron, German bessern), from *batiz- (see better (adj.)). Related: Bettered; bettering.
better (n.): 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.): 1590s, from better (v.) + -ment.
bettor (n.): 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: fem. pet name, from Bet, shortened from Elizabeth, + -y (3).
Betula (n.): genus of the birches, from Latin betula "birch," from Gaulish betu- "bitumen" (cf. 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.): Old English betweonum "between, among, by turns," Mercian betwinum, from bi- "by" (see be-) + tweonum dative plural of *tweon "two each" (cf. 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.): 1760, a jocular formation, perhaps coined by Horace Walpole, from between + -ity.
betweenness (n.): 1881, from between + -ness.
betwixt (prep., adv.): 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: fem. proper name, from Hebrew be'ulah "married woman," fem. past participle of ba'al "he married" (see baal).
bevel (adj.): 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.