The Collaborative International Dictionary of English v.0.48:

Hydrocyanic \Hy`dro*cy*an"ic\, a. [Hydro-, 2 + anic: cf. F.
   hydrocyanique.] (Chem.)
   Pertaining to, or derived from the combination of, hydrogen
   and cyanogen.
   [1913 Webster]

   Hydrocyanic acid (Chem.), a colorless, mobile, volatile
      liquid, HCN, having a characteristic peach-blossom odor.
      It is one of the most deadly poisons. It is made by the
      action of sulphuric acid on yellow prussiate of potassium
      (potassium ferrocyanide), and chemically resembles
      hydrochloric and hydrobromic acids. Called also prussic
      acid, hydrogen cyanide, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hydrocyanic acid
    n 1: a solution of hydrogen cyanide in water; weak solutions are
         used in fumigating and in the synthesis of organic
         compounds [syn: hydrocyanic acid, prussic acid]