troth
Concise Oxford English Dictionary © 2008 Oxford University Press:
troth /trəʊθ, trɒθ/
▶noun
- 1 archaic or formal faith or loyalty when pledged in a solemn agreement or undertaking.
- 2 archaic truth.
– phrases
pledge (or plight) one's troth make a solemn pledge of commitment or loyalty, especially in marriage.
pledge (or plight) one's troth make a solemn pledge of commitment or loyalty, especially in marriage.
'troth' also found in these Oxford entries:

