jacta est alea (L.), the die is cast.
jam proximus ardet Ucalegon (L.), already [the house of] our next-door neighbour, Ucalegon, is in flames.
je n'en vois pas la nécessité! (Fr.), I don't see the necessity for that! [said in reply to a man who pleaded, 'But one must live somehow'].
je ne sais quoi (Fr.), I know not what.
jet d'eau (Fr.), a jet of water.
jeu de mots (Fr.), a play on words: a pun.
jeu d'esprit (Fr.), a witticism.
jeunesse dorée (Fr.), gilded youth, luxurious young fops.
joci causâ (L.), for the sake of the joke.
judex damnatur cum nocens absolvitur (L.), the judge is condemned when the guilty man is acquitted.