I tried Wikipedia but it doesn't explain why functions are called functions.
The reason why I'm asking is because this word just doesn't make any sense for what it does.
If we were to reinvent all the words that don't make sense, and they become normal in a given culture, communication would be far more efficient and clear.
Best Answer
This is what OED has to say about the origin:
Acta Eruditorum can be found here, but it's in Latin.
A quote taken from the website Earliest Known Uses of Some of the Words of Mathematics indicates that there is an earlier use by Leibniz, in 1673: