An Etymological Dictionary of Astronomy and Astrophysics

In trigonometry, the function of an acute angle of a right triangle represented by the ratio of the opposite side to the hypotenuse.

Etymology (EN): Greek mathematicians were not aware of the advantages of sine and instead used chord. The invention of this function is a great Indian contribution. It seems that Aryabhata (c. AD 500) was the first who coined a term in Skt. for this concept: árdha-jiyā- “half chord,” which was later shortened to jiyā- “chord.” This Skt. word was subsequently loaned in Ar. and corrupted to jayb (جیب). Later on Gerard of Cremona (1114-1187),
who translated Ar. scientific texts in L., took the corrupted Skt. form jayb for Ar. jayb “pocket, bundle, bosom, fold,” and translated it to L. sinus “pocket, fold, curve.” The Skt. jiyā- “chord, bow-string;” is cognate with Av. jiiā- “bow-string;” Pers. zeh “chord, string;” → zij “astronomical table;” PIE base *g^whi- “thread, tendon” (from which derive also Gk. bios “bow;” L. filum “thread;” Russ. žca “thread”).

Etymology (PE): Sinus loanword from Fr., as above.