means a fixed expression whose overall meaning can't be worked out from the individual words, peculiar to a particular language or group.

from From Greek idiōma, "a peculiarity, a property of one's own," built on idios, "private, one's own, peculiar" — the same idios that gives us idiot (originally just a private citizen) and idiosyncrasy. It traveled through Latin and French before settling into English around the late 1500s, carrying its core sense intact: an idiom is language that belongs to itself, a phrasing private to one tongue and baffling to all others.