I don't think "notability" has anything
to do with it. Is "argue" a
non-notable word? What about "of" or "notable" or "many"
or
"describe"?
The point is, encyclopedias are supposed to be about concepts.
Dictionaries are supposed to be about words.
It's the notability of the word as a concept that is the issue.
Generally a word is only important because of the concept it
represents. Occasionally, it is important in its own right. I think
"thou" is a good example of a notable word, I'm not entirely sure,
though - I fear its notability stems (partially) from the fact that it
is no longer used, which isn't really a good measure of notability,
it's more a measure of interest. Should we be writing articles about
words simply because they are interesting? Maybe, maybe not...
A dictionary defines words, and sometimes gives their etymologies. It
doesn't usually discuss the significance of the word in the culture of
the people using it, for example. That is more suited to an
encyclopedia. For example, the fact that a group of people use the
same word to refer to two seemingly distinct concepts is something
that would simply result in two sections to the definition in a
dictionary, it could spawn several paragraphs of discussion in an
encyclopedia.