The word "unchastest" implies impurity or immodesty, so some possible antonyms could include words like "chaste," "pure," "virtuous," "modest," or "innocent." These antonyms describe qualities that are opposed to impurity or indecency, and instead suggest a sense of moral uprightness, modesty, or virtue. Other possible antonyms could include words like "decent," "proper," "respectable," or "morally sound," all of which suggest a sense of moral or social propriety that is opposed to the idea of being unchaste. Ultimately, the choice of antonyms will depend on the context in which the word is being used and the particular shades of meaning that are being conveyed.