Pertaining to consisting of, or due to quicksilver; like the god Mercury; active; flighty; fickle; changeable; pertaining to trade or trading.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (adjective) (adj.) (adjective)
Active; fickle; light- hearted; as, mercurial youth; pertaining to, made of, or caused by, mercury, or quicksilver.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
Active; sprightly; versatile; consisting of or containing mercury or quicksilver.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (adjective) (adj.) (adjective)
Having the qualities said to belong to the god Mercury: active: sprightly: containing or consisting of mercury.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (adjective) (adj.) (adjective)
Sprightly; vivacious; containing mercury.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (adjective) (adj.) (adjective)

Usage examples:

Hawksworth's mercurial relationship with the Moghul and his experiences at the Moghul's court were re- created in part from the letters and diaries of William Hawkins and those of his successor, Sir Thomas Roe.
- The Moghul by Thomas Hoover
The manner in which his ardent, imaginative, and mercurial nature was controlled by a powerful judgment, and directed by an acute sagacity, is the most extraordinary feature in his character.
- The Life and Voyages of Christopher Columbus (Vol. II) by Washington Irving
The mercurial youth seems to have no idea that within an hour he will be jeopardizing his head by engaging in a foolhardy enterprise which runs counter to the whole policy of the Turkish Empire.
- The Slaves of the Padishah by Mór Jókai