This mixture of two different acids was an important substance in the practice of alchemy.
Aqua regia (or alternatively aqua regalis) translates from Latin as 'King's water' and is comprised of both hydrochloric and nitric acid. The substance was used by alchemists as an effective way to dissolve gold, which was resistant to other acidic compounds.
This ability to dissolve gold is a possible symbolic originator of the liquid's name, as gold has often been associated with royalty, especially kings, in many capacities. In fact, rotated 45 degrees to the right, this symbol bears a strong resemblance to the famous 'orb' we sometimes see in portraits of monarchs.