From Middle English trouthe, trowthe, variant of treouthe, treuthe, from Old English trēowþ, trīewþ (“truth, veracity; faith, fidelity; pledge, covenant”), from Proto-Germanic *triwwiþō (“promise, contract”), equivalent to true + -th. More at truth.
troth (plural troths)
- (archaic) an oath, promise, or pledge
1909, Daniel Bussier Shumway (translator), “Adventure XVI”, in Nibelungenlied:
- Hagen of Troneg now foully broke his troth to Siegfried.
- specifically, a promise or pledge to marry someone
- the state of being thus pledged; betrothal, engagement
- 1893, Henry James, Collaboration 
- Vendemer’s sole fortune is his genius, and he and Paule, who confessed to an answering flame, plighted their troth like a pair of young rustics or (what comes for French people to the same thing) young Anglo-Saxons.
- 1826, James Fenimore Cooper, The Last of the Mohicans
- I did therefore what an honest man should - restored the maiden her troth, and departed the country in the service of my king.