discreetyoudaoicibaDictYouDict[discreet 词源字典]
discreet: [14] Discreet and discrete [14] are ultimately the same word. Both come from Latin discrētus, the past participle of discernere ‘distinguish’ (source of English discern). Discrete was borrowed direct from Latin, and retains its original meaning more closely: ‘distinct, separate’. The Latin abstract noun formed from the past participle, discrētiō (source of English discretion [14]), developed the sense ‘power to make distinctions’.

This fed back into the adjective, giving it the meaning ‘showing good judgment’, the semantic guise in which English acquired it from Old French discret. This was usually spelled discrete too until the 16th century, when discreet (based on the -ee- spelling commonly used in words like sweet and feet which rhymed with discrete) became the established form for the more widely used sense ‘judicious’.
=> certain, discern, discrete, secret[discreet etymology, discreet origin, 英语词源]
discreet (adj.)youdaoicibaDictYouDict
mid-14c., "morally discerning, prudent, circumspect," from Old French discret "discreet, sensible, intelligent, wise," from Latin discretus "separated, distinct," in Medieval Latin "discerning, careful," past participle of discernere "distinguish" (see discern). Meaning "separate, distinct" in English is late 14c.

Spellings discrete and nativized discreet co-existed until after c. 1600, when discreet became the common word for "careful, prudent," and discrete was maintained in philosophy, medicine, music and other disciplines that remembered Latin and made effort to obey it. Related: Discreetly.