capricious; fickle; having rapidly changing emotions
He is a mercurial character: you never know what his state of mind will be from hour to hour.
Mercurius (the planet#god Mercury. Those born under this planet were supposed to be sprightly, volatile, #quick, like the god himself.) (Latin)
Mercury level in the blood pressure testing device fickles a lot when measuring blood pressure. Mercury liquid if left on a surface has high fluidity. Hence quick and fickle. Also, mercury is VOLATILE (another synonym of mercurial)