The line between regular phrases, phrases that we put together word by word, and idiomatic phrases, those we memorize whole (fly off the handle, climb the walls, straignt and narrow) can be fuzzy. When my dentist suggested I might need a root canal, I began wondering where this usage came from. (Which remainds me, I should call him and ask if he said anything important after making that announcement).
A root canal is an operation, not a thing as canal implies. It is an operation on the root canal(s) of a tooth but is not a canal itself. How do we understand this kind of misusage?
Well, we have to memorize an enormous amount of phrases that on their face make no sense. A red cap is not a cap, a quick study is not a study, nor is an egghead (just) a head. A root canal is an operation ON a root canal while a quick study is someone who studies (and learns) quickly.
These phrases and words are all idiomatic. Unlike grammar and the word lexicon which are processed in the left hemisphere of the brain, idiomatic expressions are processed in the right hemisphere. This makes sense given the left hemisphere specializes in analytic processing and the right hemisphere, wholistic processing. Idioms are processed as wholes with a single meaning.
The distinction between analytic speech constructs like “the mosquitor flew off the handle” and wholistic (synthetic) constructs like “my aunt flew off the handle” makes it clear that speech is something of a science and an art: a process of putting together atomic constituents (words) and whole preprocessed chunks.
I find mulling over the mysteries and complexities of speech and language much more rewarding than contemplating root canals or even counterfeit toothpaste, which can cause them.