*idiomatical*

{ } a. [ Gr. 'idiwmatiko`s. ] 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 idiom{3}; having a meaning that is peculiar to itself and not predictable from general rules. [PJC]