The word "idiom" comes from the Greek word "ἴδιος" (idios), meaning "one's own" or "peculiar." It was originally used in the 16th century to describe a particular style of language or a specific dialect.
Over time, the meaning evolved to refer to a phrase or expression that is peculiar to a particular language or group of people. This is because idioms often have a meaning that cannot be understood from the literal meaning of the words. For example, "kick the bucket" means "to die," but you wouldn't literally kick a bucket to die.
So, the origin of the word "idiom" is rooted in the Greek concept of something unique or distinctive, and it reflects the special nature of these non-literal expressions in language.