Picture a bead of silver-coloured metal that will not stay still. Touch it and it splits, rolls, and is somewhere else before your finger lands. Old English named it living silver for exactly that reason. Every other use of the word can be read as that bead, lent out: a mood, a mind, a smile or a scene that has moved before you can take hold of it.
From Old English cwicseolfor, literally living silver: quick here is the old word for living, not for fast. The name records how the metal behaves rather than what it is made of.
來自古英文 cwicseolfor，字面是「活的銀」。這裡的 quick 是古英文的「活著的」，不是「快」。也就是說，這個名字記錄的是這種金屬的動作，而不是它的成分。
Wiktionary entry for quicksilver, supplied in this evidence packet (kaikki:3f6f69a190036ae4:499410659)
Two branches
會自己滾動的金屬
這一支就是畫面本身：真的那顆銀珠子。
noun · C2
The metal mercury: a heavy, shiny, silver-coloured metal that is liquid at normal temperatures, so a drop of it rolls and slides instead of holding still. This is the older English name for it; the usual name today is mercury.
水銀（汞）的舊稱
借這顆珠子形容變化
把「抓不住」這個動作從金屬身上取下來，蓋到人或事情上。
noun · C2
Used in writing to talk about something that changes or moves very fast and in a way you did not expect, by comparing it to the liquid metal. It appears most often in the phrase like quicksilver, about a person's mood, thoughts, or movements.
如水銀般變化極快