Derisive sarcasm isn't useful here.

Definitions are still a useful tool and help clarify the semantic field. Dictionaries are a project that imply that meaning is dependent and contextual. Dictionaries attempt to capture it, for now. A word's meaning depends upon its part of speech and can mean different things when present in different parts of speech i.e., row. Homonyms, of which contranyms like anxious and cleave are a subset of, can even exist in the same part of speech. "A bat flew past me" is a meaningful statement, but we have deferred it's meaning until context reveals what type of bat. It could literally be either.

Etymologies can help understand how this happens. Or their transformation can be lost. Languages change. The word "ephemera" has nothing to do with fevers. Original meaning is not the supreme meaning. Connection to the original does not confer primacy. "Cleave" means to "stay close to" and "split apart". When you look at how the same word from two different non-English sources enter English at two different times, you see how a contranym can emerge.

The meaning of a word is open to change from social circumstances. Just because it used to mean something like a one day fever doesn't mean it still means that nor does it mean that it's connection is either obvious, tracable, or necessary.

A fixed meaning has to be divorced from people and it's use. Language is a reflection of the people who use it. Meaning has several points of instability. Only context can fasten it. Context is the only way meaning is reveal despite our anxious anticipation for its stability. We are ahead of the meaning when we prematurely seek it's stability, clarity, and certainty. And when contranyms allow for double meaning, it can be an invitation to play. And is anything more human than that?