quicksilver (n.)

common popular name of the metal mercury, Old English cwicseolfor, literally "living silver," so called for its mobility, translating Latin argentum vivum (source also of Italian argento vivo), literally "living silver;" so called from its liquid mobility. See quick (adj.) + silver (n.). Similar formation in Dutch kwikzilver, Old High German quecsilbar, German quecksilber, French vif-argent, Italian argenta viva.

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Definitions of quicksilver from WordNet
quicksilver (n.)
a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures;
Synonyms: mercury / hydrargyrum / Hg / atomic number "
quicksilver (adj.)
liable to sudden unpredictable change;
a quicksilver character, cool and willful at one moment, utterly fragile the next
Synonyms: erratic / fickle / mercurial