nitrohydrochloric : Idioms & Phrases

• noun a yellow fuming corrosive mixture of nitric and hydrochloric acid that dissolves metals (including gold)
  aqua regia.
  

WordNet

• a mixture of nitric and hydrochloric acids, usually in the proportion of one part of the former to three of the latter, and remarkable for its solvent action on gold and platinum; called also nitromuriatic acid, and aqua regia.

Webster 1913