quicksilver n.

Middle English quik, from Old English cwic "living, alive, animate, characterized by the presence of life" (now archaic), and figuratively, of mental qualities, "rapid, ready," from Proto-Germanic *kwikwaz (source also of Old Saxon and Old Frisian quik, Old Norse kvikr "living, alive," Dutch kwik "lively, bright, sprightly," Old High German quec "lively," German keck "bold"), from PIE root *gwei- "to live." Sense of "lively, active, swift, speedy, hasty," developed by c. 1300, on notion of "full of life."

NE swift or the now more common fast may apply to rapid motion of any duration, while in quick (in accordance with its original sense of 'live, lively') there is a notion of 'sudden' or 'soon over.' We speak of a fast horse or runner in a race, a quick starter but not a quick horse. A somewhat similar feeling may distinguish NHG schnell and rasch or it may be more a matter of local preference. [Carl Darling Buck, "A Dictionary of Selected Synonyms in the Principal Indo-European Languages," 1949]

Of persons, "mentally active, prompt to perceive or respond to impressions" from late 15c. Of an action, process, etc., "done in little time," 1540s. Also in Middle English used of soft soils, gravel pits, etc. where the ground is shifting and yielding (mid-14c., compare quicksand). Also in Middle English "with child, in an advanced state of pregnancy" (when the woman can feel the child move within). Also formerly of bright flowers or colors (c. 1200).

As an adverb, "quickly, in a quick manner," from c. 1300. To be quick about something is from 1937. Quick buck is from 1946, American English. Quick-change artist (1886) originally was an actor expert in playing different roles in the same performance of a show. Quick-witted is from 1520s.

"rare white metal, harder than gold, softer than copper, valued for its luster and malleability," Middle English silver, from Old English seolfor, Mercian sylfur, Northumbrian sulfer, "the metal silver; silver coin, money," from Proto-Germanic *silabur- (source also of Old Saxon silvbar, Old Frisian selover, Old Norse silfr, Middle Dutch silver, Dutch zilver, Old High German silabar, German silber "silver; money," Gothic silubr "silver"), which is of uncertain origin.

It seems to be Germanic/Balto-Slavic (source also of Old Church Slavonic s(u)rebo, Russian serebro, Polish srebro, Lithuanian sidabras "silver"), but has long been presumed to be a Wanderwort (a loan-word that has spread among several languages) displacing the usual IE word for the metal (represented by Latin argentum; see argent).

Basque zilharr "silver" usually is considered a loan-word from West Germanic, but the Germanic form lately has been compared to old Celtic words used in Spain, and because the rest of Celtic uses the argentum word, this suggests the borrowing might be in the other direction, and Germanic word might be from "a Hispano-Celtic innovation due to an Iberian donor language. In this connection, the old comparison of Basque zilharr is attractive" [Boutkan].

From c. 1300 as "articles, plates, etc. of silver, silverware." As a color name from late 15c. Chemical abbreviation Ag is from Latin argentum "silver."