hydrocyanic : Idioms & Phrases

• noun a solution of hydrogen cyanide in water; weak solutions are used in fumigating and in the synthesis of organic compounds
  prussic acid.
  

WordNet

• (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.

Webster 1913