betroth

From Middle English bitrouthen, bitreuthen (“of a man: to pledge to marry; to give (a woman) in marriage, arrange the marriage of”), from bi- (prefix forming transitive verbs from nouns) + trouth, treuthe (“faithfulness, fidelity, specifically marital fidelity; promise, undertaking, specifically a promise of marriage; truth; etc.”) (from Old English trīewþ, trēowþ (“fidelity; good faith, honour; assurance of good faith, covenant, troth; truth”), from Proto-Germanic triwwiþō (“contract; promise”), from Proto-Indo-European dóru (“tree; hence, firm, hard; faithful, true”)), possibly modelled after Old English trēowsian (“to pledge oneself; to prove oneself to be true”)). By surface analysis, be- + troth.