Some words I use so rarely that when I finally put them in a sentence I'm unsure what exactly they mean, though I feel vaguely like they belong there. This happened yesterday when I was writing an email. The sentence went something like this: "After deciding that the critique is trenchant, the editors should send it out for review." What I meant was some combination of "relevant and supported by argument" or, more colloquially, "spot on", and I suppose the context of the mail will make that clear. But then I looked it up and (as I had suspected) it turns out that "trenchant" means something quite different: "1 (of a style or language etc.) incisive, terse, vigorous 2 archaic or poet. sharp, keen" (Concise Oxford Dictionary.) My use of the word arguably cuts across the two senses: something like "incisive and keen". Etymologically, "trenchant" can be traced back, through "truncate" to the Latin for "maim". A trenchant critique is one that seriously wounds the object, not just hits it squarely, as I had hoped.
There's a song that uses the word correctly, though without teaching us its meaning:
Once I said, and I quote, I just read this thing that you wrote in college,
A trenchant critique of anthropology being accepted as a social science
And not the art of educated observation.
If one did not look it up, one might take "trenchant" to mean simply "solid", "on target", "relevant" or "timely". It means "cutting" but not quite "scathing", assertive but not yet shrill.
Another word that is sometimes used as though we know what it means, but haven't quite understood, is "salient", which means "jutting out; prominent; conspicuous; most noticeable". Here, again, because of the way we hear the word used, we sometimes think it is merely a term of praise, not also an objective characterization. We talk of making "salient points"* in a discussion or constructing a "salient argument". Again, our impression is that the word simply means "relevant" or "good" or "valid". A salient point or argument is actually one that "jumps out at you" or "sticks out". The etymology is useful here: from the Latin for "leap" (saliere).
"If you plan to use 'colubriform' in public," Hugh Kenner warns, "you'd best devote fifteen minutes to making sure it really means what you want it to." The same goes for words like "trenchant" and "salient". Look them up in a dictionary and try to find out how they are used by people writing in your field. Use Google Scholar and try to notice how these words appear in context. Sometimes they are, of course, used incorrectly even by scholars. But often, with the dictionary definition in mind, you can suddenly see that they don't just mean what you thought they meant. When Jones says of Smith's argument that it is "salient", he doesn't mean that it is good (though he often thinks so too). He means the argument leaps off the page, that it stands out from the other arguments that might be cited.
*Note that this can have a technical meaning in statistical analysis too. That's not the sense I'm after here.