"Before her coming, Maggie was sad, but tranquil, and as she believed herself, free -- Marie left her exalted, miserable, and bound by a solemn promise to hold fast her
troth, in defiance of parents, friends, evil reports, the world!"
--"Nobody to Blame" by Marion Harland,
Godey's Lady's Book and Magazine, April 1864
(I found a bound copy of several issues of
Godey's in my basement and have been flipping through the dusty pages.)
troth = pledged faithfulness or word, fidelity
"betrothed" is related I believe. And if you "keep your word" about something, you have decided to "plight your troth". ("Plight" means to promise there, not that you're putting your fidelity in a pickle.)