Word 'idiom' s sudden jump to current meaning from its original

Question

I am learning English steadily in a way to discern its culture context behind the vocabulary.

In my opinion, it's a big jump of word 'idiom' from its orginal to current meaning

In Etymology Dictionary, it specify:
from idioumai "to appropriate to oneself," from idios "personal, private," properly "particular to oneself."

The current meaning in Oxford Dictionaries is
group of words established by usage as having a meaning not deducible from those of the individual words (e.g. over the moon, see the light). synonyms with set phrase, fixed expression, phrase.

It's a long stride from 'private' to 'fixed expression'.

How to interpret it?

Answer 1

Well, one way that, I think, could help you deal with this is to think of an idiom as a bunch of words everyone is very well familiar with that, when taken together as a whole, have a unique, particular to that idiom, meaning. In other words, what an idiom really does is it appropriates words in everyday use for its own, private, purpose.

By the way, you will be surprised to know that the words idiot and idiom trace their roots back to the same Greek word ἰδιώτης (read idiotos) which meant a private or common person (or simply a commoner) as opposed to the elite.

Answer 2

According to your research idioumai means to appropriate something---to take and keep it (perhaps to steal it).

An idiom is a set of words that have one literal meaning, but people have taken them and used them for another purpose enough that the original, literal, meaning is no longer their primary definition (when seen together). In a sense these people have stolen the words or phrase from the original language in order to do something different with them.

Note that the essential aspect of an "idiom" is not that it is a fixed set of words, but that these words have been taken out of the original (literal) language and repurposed for something else.