The definition of mercurial is changeable or fickle.(adjective)
Someone who can get mad in the blink of an eye is an example of someone who would be described asmercurial.
See mercurial in Webster's New World College Dictionary
Origin: ME < L mercurialis
See mercurial in American Heritage Dictionary 4
Origin: Middle English, of the planet Mercury
Origin: , from Latin mercuriālis, of the god or planet Mercury
Origin: , from Mercurius, Mercury.
Learn more about mercurial