BONA FIDE
“Bona fides” is sometimes treated as a plural in English, but this would sit oddly with a Latin speaker. Bona fides (good faith) is singular.
Fides is the root of fidelity. Infidelity is unfaithfulness. A hi-fidelity system reproduces sound with faithful accuracy.
UK coins carry the inscription Fid. Def., or sometimes FD. It is short for fidei defensor, or “Defender of the Faith”, a title granted to Henry VIII in 1521 for having a pop at Martin Luther. Fidei is the genitive singular, of the faith. Fide is the ablative form, so bona fide means with good faith or in good faith.
Bona fide and its rarer opposite, mala fide, are important terms in commerce and law. Bona fide has also slipped into general English usage to suggest qualifications or qualifications. In Ireland, bona fide travellers were people who could prove that they had travelled three miles or more, and were therefore entitled to pub refreshment outside normal licensing hours. Until recently you could still see ruby-faced men in a train station bar on a Good Friday afternoon, dripping porter onto the cheapest available ticket. Bona fide travellers? Maybe.