Does the derivation of a word tell you what it means?

The short answer to this question is "no". Although the study of the history and origins of words has merit of its own, it cannot help people write better. The knowledge that a word is descended from another word in an ancient language is no help in deciding how best to use it. The fact that style comes from a word meaning a pointed instrument is of no practical use to the person needing to know whether it is the right word to use in a particular context. Similarly, the fact that presently meant immediately in Elizabethan English is a dangerous distraction to someone trying to use it properly in modern times.

Though we may lament the fact that the meaning of aggravate has changed to encompass irritate as well as make worse, we ignore such changes at our peril. Similarly, the person who does not check whether someone saying mistrust actually means distrust runs the risk of misunderstanding. Someone in the computing-industry who asks how a piece of software performs isn't interested in how well it does its job, but just how fast it runs. (This probably comes from the euphemistic use by the motor-industry of performance in high performance car.) Banks need to understand that their customers refer to a cheque which has cleared as having been cashed. For many people, a couple of things isn't necessarily two of them, while for others a discussion is a monologue.