Trig reciprocal function nomenclature? The fact that the reciprocal of $\sin\theta$ is $\csc\theta$, and the reciprocal of $\cos\theta$ is $\sec\theta$ messed with my head for the longest time when I was taking trig. Why are the functions named this way, when an alliterative scheme would seemingly be more sensible? While I'm at it, what's the reason for choosing the names sine, cosine, and tangent? The words sinusoidal and tangental come to mind, but perhaps these words could have come from the function names, not the other way around.

Here is a nice explanation of the origin of all these names. In summary:

Most of the words come from Latin descriptions of the geometry involved. Sine comes from the Latin word 'sinus', tangent from the Latin 'tangens', and secant from the Latin 'secans'. The origin of the co-functions actually makes quite a lot of sense; cosine was originally co-sine, referring to the sine of the complementary angle. Similarly, cotangent and cosecant are the tangent and secant of the complementary angle.

I recommend reading the webpage if you want to know more detail, it's quite interesting.