hydrargyrum (n.)

"mercury, quicksilver," 1560s, from Latin hydrargyrus, from Greek hydrargyros "quicksilver" (as prepared artificially from cinnabar ore; native quicksilver was argyros khytos "fused silver"), from hydr-, stem of hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet") + argyros "silver" (from PIE root *arg- "to shine; white," hence "silver" as the shining or white metal). Hence the chemical abbreviation Hg for the element mercury.