While there is no private language (Wittgenstein), every natural language abounds in idioms (from the Greek word meaning private) – expressions whose specific usage is not derivable from the logos of the language. In fact any particular usage of an expression may occupy any position along a spectrum running from fully public to fully private. Idioms, jargon and slang are perhaps signs of the propensity of cultures to articulate themselves as subcultures, and of people to identify themselves as members of a group by adopting the group’s articulation habits.
Looking at language itself is like looking at a mirror rather than at the reflection in a mirror, or looking at a window rather than through it: the phenomenon of language ceases to be transparent and we focus on its dynamics rather than those of “the world.” This can raise its automatic functions into consciousness and restore the immediacy of the habits we have taken for granted.
For instance, we can look at familiar idioms which we normally use automatically, and try to trace their peculiar logic back to the root. Consider the English expression “back and forth”: why is it not “forth and back”? Do we normally imagine that kind of motion as beginning with the return? Maybe we do: perhaps an irreversible “going forth” is the default kind of motion, as it were, and only when this is interrupted by a “coming back” do we notice a distinctive pattern – which we then call “back-and-forth.”
But then why do we say that someone tumbles “head over heels”? Taken “literally,” this expression would be equivalent to “upside up”; “upside down” (or “downside up”) would be better expressed as “heels over head.” Well, maybe the idiom is dominated by the dynamic (rather than the static positional) sense of over – as in “fall over,” “turn over” etc. – and we simply ignore the order of the nouns.
There’s an element of chance in any evolutionary process, including historical changes of word meaning; idiomatic and conventional expressions are full of ‘frozen accidents.’ Some accidents are more likely, more ‘motivated,’ than others (Lakoff 1987, Sweetser 1990), but all are unruly to some degree, as is life itself.