Observant

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late Middle English (as a noun): from French, literally ‘watching’, present participle of observer (see observe).

wiktionary

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Borrowed from French observant

etymonline

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observant (adj.)

c. 1600, "attentive in perceiving or taking notice, characterized by good powers of observation," also "attentive in observing what is prescribed or required" (a law, custom, etc.), from observe + -ant, or else from French observant, past participle of observer (see observance). In reference to Judaism, "strict in acting in accordance with precepts," from 1902. As a noun from late 15c. Related: Observantly; observantness.