Same word as quicksilver:

arbitrary, capricious, changeable, cheating, coquettish, double-crossing, faithless, fitful, flighty, frivolous, inconstant, irresolute, lubricious, mercurial, mutable, quicksilver, sneaking, temperamental, ticklish, two-timing, unfaithful, unpredictable, unstable, unsteady, untrue, variable, volatile, whimsical, yo-yo