Source : Webster's Revised Unabridged Dictionary (1913)

Idiomatic \Id`i*o*mat"ic\, Idiomatical \Id`i*o*mat"ic*al\, a.
   [Gr. ?.]
   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.

Source : WordNet®

idiomatically
     adv : in an idiomatic manner; "he expressed himself idiomatically"