On Fri, May 11, 2012 at 9:59 AM,  <[log in to unmask]> wrote:
> Yes, I've mentioned this before on the list. We seem to use
> "idiom" to mean two different things.
> 1. Things which make no grammatical sense: "I will try and
> go to the party."
> 2. Things which are grammatical, but make no physical
> sense: "It was raining cats and dogs."

Those with the time and the inclination may find it useful and to look
into Construction Grammar, which was in part created to address this
very word <-> idiom (in both senses) issue.  For real fun, there is
William Croft's Radical Construction Grammar (cached PDF link near the
top right),

which views words and syntax (as traditionally understood) as simply
to points on a continuum.  I've been slowly reading an introduction
to Langacker's Cognitive Grammar, and he takes several opportunities
to mock the question, apparently heard in linguistics seminars, "is that
in the lexicon or the syntax."

Whether or not one is inclined to accept the various CxG approaches,
they seem like productive fictions for thinking about your conlang's
vocabulary and "idioms" if you favor a naturalistic result.