troth

From Middle English trouthe, trowthe, a variant of treuth, treuthe, trewthe (“allegiance, fidelity, faithfulness, loyalty; oath, pledge, promise; betrothal or marriage vow; betrothal; honour, integrity; holiness, righteousness; confidence, trust; creed, faith; fact, reality, truth”), from Old English trēowþ, trīewþ (“truth, veracity; faith, fidelity; covenant, pledge”), from Proto-Germanic *triwwiþō (“contract; promise”), equivalent to true +‎ -th (abstract nominal suffix). Doublet of truth.