Etymology

cosine

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Today it means: In a right triangle, the ratio of the length of the side adjacent to an acute angle to the length of the hypotenuse. Symbol: cos Full definition →

English sine ← Latin sinus ← Arabic جِيبَ (jība)
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The story

  1. Proto-Indo-European*ḱe?
  2. Proto-Indo-European*ḱóm
  3. Proto-Italic*kom
  4. Proto-Italic*kom-
  5. Latincon-
  6. Latinco-
  7. Englishco-
  8. Proto-Indo-European*sinos
  9. Proto-Italic*sinus
  10. Arabicجِيبَ (jība)
  11. Latinsinus
  12. Englishsine
  13. English todaycosine

From co- + sine, short for complementi sinus in Latin, meaning “sine of the complement”. Doublet of cosinus.

Etymology from Wiktionary, CC BY-SA 4.0.