/saɪn/

Borrowed from Latin sinus (“curve, bend; bosom”), a translation of Arabic جَيْب‎ (jayb, “bosom”), a misidentification of the notation جيب (j-y-b), written without vowel diacritics, standing for Arabic جِيبَ‎ (jība, “sine”), in turn from Sanskrit ज्या (jyā, “sine, chord, bowstring”) through the similar Sanskrit जीव (jīva, “sine, chord, life, existence”).