bona fides

Bona fides looks like a plural word in English, but in Latin, it's a singular noun that literally means "good faith." When bona fides entered English, it at first stayed very close to its Latin use - it was found mostly in legal contexts and it meant "honesty or lawfulness of purpose," just as it did in Latin. It also retained its singular construction. Using this original sense one might speak of "a claimant whose bona fides is unquestionable," for example. But in the 20th century, use of bona fides began to widen, and it began to appear with a plural verb in certain contexts. For example, a sentence such as "the informant's bona fides were ascertained" is now possible.