bona fides

Bona fides is from the singular Latin phrase bona fidēs , meaning “good faith,” and has the same meaning in English. But partially because its -es ending makes bona fides look and sound like a plural, it has developed the plural sense “credentials.” This plural use, although criticized by some usage guides, has been increasing in all varieties of speech and writing. The adjective bona fide (without the “s”) is from a Latin phrase meaning “in good faith, with good faith.” It was originally used adverbially in this sense, but is now mainly an adjective. The meaning “authentic, true” is a later development sometimes denounced as sloppy usage, but its use is bona fide and widespread.