troth

troth \TRAWTH\, noun:

1. Faithfulness, fidelity, or loyalty.
2. One's word or promise, especially in engaging oneself to marry.

I did therefore what an honest man should--restored the maiden her troth , and departed the country in the service of my king.
-- James Fenimore Cooper, The Leatherstocking Tales

I was wild--in troth I might go yet farther and say VERY wild, though 'twas a wildness of an innocent sort, since it hurt none but me, brought shame to none, nor loss, nor had in it any taint of crime or baseness, or what might not beseem mine honourable degree.
-- Mark Twain, The Prince and the Pauper

Troth derives from a variation of truth in certain regions of England. Over time it has taken on a distinct meaning in certain phrases