Definition:Cosine/Linguistic Note

Linguistic Note on Cosine
As with sine, the word cosine derives (erroneously, via a mistranslation) from the Latin sinus which (among other things) means fold, curve, winding or bay.

The co- prefix, as with similar trigonometric functions, is a reference to complementary angle: see Cosine of Complement equals Sine.

It is pronounced with the emphasis on the first syllable: co-sign.

$\cos x$ is voiced cosine (of) $x$, or (as written) cos $x$ (pronounced either coss or coz depending on preference).