Late 18th century; earliest use found in The Annual Register. From classical Latin idiōmat-, idiōma or its etymon Hellenistic Greek ἰδιώματ-, ἰδίωμαidiom + -ism.
Definition of idiomatism in US English:
idiomatism
nounˌɪdɪˈəʊmətɪz(ə)m
rare
An idiomatic expression.
Origin
Late 18th century; earliest use found in The Annual Register. From classical Latin idiōmat-, idiōma or its etymon Hellenistic Greek ἰδιώματ-, ἰδίωμα idiom + -ism.