In this case, I’m using idiom in its “I was a Creative Writing Major at College” sense; that is:

A speech form or an expression of a given language that is peculiar to itself grammatically or cannot be understood from the individual meanings of its elements, as in “keep tabs on”.

*credit to Wordnik.com

So my use of the word here just means “expression” or “figure of speech,” which is probably what I should have said in the first place; sorry for the confusion.

Edit: a grammatical correction no one but me would probably notice or care about.