GNU Collaborative International Dictionary of English

Found one definition

1.       
    Idiomatic (?), Idiomatical (?),  a. [Gr. ἰδιωματικός.]
   1. Of or pertaining to, or conforming to, the mode of expression peculiar to a language; as, “an idiomatic meaning; an idiomatic phrase.” -- Id`i*o*mat"ic*al*ly, adv.
   
   [1913 Webster]
   
   
   2. Of or pertaining to, or of the nature of an idiom3; having a meaning that is peculiar to itself and not predictable from general rules.
   
   [PJC]