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
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?