Etymology
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betroth (v.)
c. 1300, betrouthen, "to promise to marry (a woman)," from be-, here probably with a sense of "thoroughly," + Middle English treowðe "truth," from Old English treowðe "truth, a pledge" (see truth). From 1560s as "contract to give (a woman) in marriage to another, affiance." Related: Betrothed; betrothing.
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betrothed (adj.)
1530s, past-participle adjective from betroth (v.). As a noun, in use by 1580s.
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betrothal (n.)
"act of betrothing," 1825, from betroth + -al (2). Earlier in same sense were betrothment (1580s), betrothing (14c.).
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troth (n.)
"truth, verity," late 12c., from a phonetic variant of Old English treowð "faithfulness, veracity, truth;" see truth, which is a doublet of this word. Restricted to Midlands and Northern England dialect after 16c., and to certain archaic phrases (such as plight one's troth). Also see betroth.
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*deru- 

also *dreu-, Proto-Indo-European root meaning "be firm, solid, steadfast," with specialized senses "wood," "tree" and derivatives referring to objects made of wood.

It forms all or part of: betroth; Dante; dendrite; dendro-; dendrochronology; dour; Druid; drupe; dryad; dura mater; durable; durance; duration; duress; during; durum; endure; hamadryad; indurate; obdurate; perdurable; philodendron; rhododendron; shelter; tar (n.1) "viscous liquid;" tray; tree; trig (adj.) "smart, trim;" trim; troth; trough; trow; truce; true; trust; truth; tryst.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dru "tree, wood," daru "wood, log, timber;" Greek drys "oak," drymos "copse, thicket," doru "beam, shaft of a spear;" Old Church Slavonic drievo "tree, wood," Serbian drvo "tree," drva "wood," Russian drevo "tree, wood," Czech drva, Polish drwa "wood;" Lithuanian drūtas "firm," derva "pine, wood;" Welsh drud, Old Irish dron "strong," Welsh derw "true," Old Irish derb "sure," Old Irish daur, Welsh derwen "oak;" Albanian drusk "oak;" Old English treo, treow "tree," triewe "faithful, trustworthy, honest."

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handfast (v.)

"betroth (two people), bind in wedlock; pledge oneself to," early 12c., from Old English handfæsten and cognate Old Norse handfesta "to pledge, betroth; strike a bargain by shaking hands;" for first element see hand (n.); second element is from Proto-Germanic causative verb *fastjan "to make firm," from PIE *past- "solid, firm" (see fast (adj.)). Related: Handfasted; handfasting. The noun in Old English was handfæstung.

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fasten (v.)
Old English fæstnian "make fast, make firm, fix, secure," also "ratify, betroth, confirm," from Proto-Germanic *fastinon "to make firm or fast" (source also of Old Frisian festnia "to make firm, bind fast," Old Saxon fastnon, Old High German fastnion, German festnen, Old Norse fastna "to pledge, betroth"), from PIE *fast "solid, firm" (see fast (adj.)). Related: Fastened; fastening.
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fiance (n.)
"man to whom one is betrothed," 1864 (by 1826 as a French word in English), from French fiancé, past participle of fiancer "to betroth" (see fiancee). Borrowed earlier in Middle English as "confidence, trust; word of honor."
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wed (v.)

Old English weddian "to pledge oneself, covenant to do something, vow; betroth, marry," also "unite (two other people) in a marriage, conduct the marriage ceremony," from Proto-Germanic *wadja (source also of Old Norse veðja, Danish vedde "to bet, wager," Old Frisian weddia "to promise," Gothic ga-wadjon "to betroth"), from PIE root *wadh- (1) "to pledge, to redeem a pledge" (source also of Latin vas, genitive vadis "bail, security," Lithuanian vaduoti "to redeem a pledge"), which is of uncertain origin.

The sense has remained closer to "pledge" in other Germanic languages (such as German Wette "a bet, wager"); development to "marry" is unique to English. "Originally 'make a woman one's wife by giving a pledge or earnest money', then used of either party" [Buck]. Passively, of two people, "to be joined as husband and wife," from c. 1200. Related: Wedded; wedding.

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fiancee (n.)
"woman to whom one is betrothed," 1844 (1837 as a French word in English), from French fiancée, fem. of fiancé, past participle of fiancer "to betroth," from fiance "a promise, trust," from fier "to trust," from Vulgar Latin *fidare "to trust," from Latin fidus "faithful" from the same root as fides "faith" (from PIE root *bheidh- "to trust, confide, persuade"). It has all but expelled native betrothed. The English verb fiance, now obsolete, was used c. 1450-1600 for "to engage to be married."
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