/ mɜːˈkjʊəriəl; NAmE mɜːrˈkjʊr-/ adjective
1.
( literary ) often changing or reacting in a way that is unexpected
SYN volatile :
Emily's mercurial temperament made her difficult to live with.
2.
( literary ) lively and quick :
a brilliant, mercurial mind
3.
( technical ) containing mercury
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WORD ORIGIN
late Middle English (referring to something of the planet Mercury): from Latin mercurialis relating to the god Mercury, from Mercurius Mercury. Senses 1 and 2 date from the mid 17th cent.