Letting the Idioms out of the bag

Let the cat out of the bag / Not my cup of tea / Spill the beans / Clear as mud.

The Story

Idioms have been on my mind lately.  For example, I'd been wondering how actually to define an idiom, and what idioms students really need to know.  I also had vague recollections of looking through an Academic English textbook a couple of years ago that insisted students be taught that words such as 'affluent' are idioms, because of the Latin etymology from ad+fluere = flowing towards.  While this may be linguistically accurate, it's hard for me to imagine any student being enough of a language nerd to be helped by such an approach.  Anyway, isn't all speech necessarily figurative, when we decide that a sequence of sounds represents meaning?