Does it ever happen to you that you find a piece of paper on your bedside table in the morning, with some weird scribbles, and you recall that you had this brilliant realization in a dream and wrote it down for posterity…? This is what I just found:
There are more grammatically correct combinations of words in a language, than semantically meaningful sentences. And there are also meanings that we have no words to express. So to be more efficient, we assign these sentence-less meanings to the meaning-less sentences, and presto, idioms!
I am by far not the first one to point this out — Douglas Adams wrote a whole book, the Deeper Meaning of Liff, about this topic! (“A dictionary of things that there aren’t any words for yet; all the words listed are toponyms [place names], and describe common feelings and objects for which there is no current English word.”) Very funny book by the way. My copy came with an added German “translation” (brilliantly mapping the words to placenames from the German language region), and includes a weird “preface” about how the German translator slowly goes insane. (For example he goes to a bookshop demanding to buy an index where all German placenames are sorted alphabetically by their last syllable or something.) ;-)