Sinus

google

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late Middle English (in the medical sense): from Latin, literally ‘a recess, bend’.

wiktionary

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Unadapted borrowing from Latin sinus(“a bent surface, curve, hollow”).  Doublet of  sine. 

etymonline

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sinus (n.)

"hollow curve or cavity in the body," early 15c., from Medieval Latin sinus, from Latin sinus "bend, fold, curve, a bent surface; a bay, bight, gulf; a fold in land;" also "fold of the toga about the breast," hence "bosom," and figuratively "love, affection, intimacy; interior, inmost part;" of unknown origin.