IPA: /ˌɪdiˈɒmətɪk/
KK: /ˈɪdiˌɑːmætɪk/
Relating to expressions or phrases that are natural and common in a particular language, often not easily translated word-for-word into another language.
She speaks idiomatic English, making her sound like a native speaker.
Idiomatic → It is formed from "idio-" (from Greek "idios", meaning one's own or private) and "-matic" (from Greek "matikos", meaning related to or characterized by). The word describes expressions that are characteristic of a particular language or group, often having a meaning that is not deducible from the individual words.
Think of 'one's own' ('idio-') expressions that are 'characteristic' ('-matic') of a language — that's what idiomatic means.