IPA: /ˌʌn.ɪˈdaɪ.mə.tɪk/
KK: /ʌnˌɪdiˈɑːmɪtɪk/
Not using the natural or expected phrases of a language, making it sound awkward or unnatural.
His speech was unidiomatic, making it hard for the audience to understand him.
Unidiomatic → It is formed from "un-" (meaning not) and "idiomatic" (from Greek "idioma", meaning a peculiar phrase or expression). The word "unidiomatic" means not characteristic of a particular language or idiom, indicating a lack of naturalness in expression.
Think of 'not' ('un-') being combined with a 'peculiar expression' ('idiomatic') to remember that 'unidiomatic' refers to something that does not fit the natural way of speaking.