mercurial (adj.)

late 14c., "pertaining to the planet Mercury" (see Mercury). Meaning "sprightly, volatile, quick" (1590s) is from supposed qualities of those born under the planet Mercury (they also are the qualities of the god Mercury), probably also partly by association with the qualities of quicksilver. A variant in this sense was mercurious (1590s). Related: Mercurially; mercuriality.