Quicksilver (noun)

1. The chemical element mercury, a heavy silvery-white metal that is a liquid at room temperature and pressure.
2. Something that is elusive or constantly changing.

Origin:

Middle english: from quick 'alive' + silver, the liquid metal being so named because of its fluidity and lively appearance.

Examples:

1. The quicksilver in the thermometer rose as the temperature increased.
2. Quicksilver was once used in the extraction of gold and silver from their ores.
3. Her moods were as changeable as quicksilver.
4. The quicksilver spill contaminated the river and killed the fish.
5. He was as quick as quicksilver and never stayed in one place for long.