Meaning

adjective liable to sudden unpredictable change

adjective relating to or under the (astrological) influence of the planet Mercury

adjective relating to or having characteristics (eloquence, shrewdness, swiftness, thievishness) attributed to the god Mercury

originNoun sense 1 (“(obsolete) plant known as mercury”) is from Middle English mercurial (“a plant, probably the goosefoot (Chenopodium); (possibly) dog’s mercury (Mercurialis perennis)”), from Anglo-Norman mercurial, Old French mercurial, or directly from their etymon Latin mercuriālis (“a plant, probably annual mercury (Mercurialis annua)”), short for herba mercuriālis (“(probably) annual mercury”,…

Etymology from Wiktionary, CC BY-SA 3.0