mercurial
adj.adj. describing someone who changes their mind or mood very quickly. It can also describe something that is hard to control or predict because it moves so fast.
adj. characterised by sudden, unpredictable, or rapid changes in mood, temperament, or fortune. Historically associated with the planet Mercury and the element mercury due to their perceived volatility.
He is a mercurial character who changes his plans every hour.
The politician's mercurial nature made it difficult for his advisors to predict how he would react to the new scandal.
The poet's mercurial temperament is reflected in his body of work, which ranges from deeply somber elegies to lighthearted satires within a single volume.
Noun sense 1 (“(obsolete) plant known as mercury”) is from Middle English mercurial (“a plant, probably the goosefoot (Chenopodium); (possibly) dog’s mercury (Mercurialis perennis)”), from Anglo-Norman mercurial, Old French mercurial, or directly from their etymon Latin mercuriālis (“a plant, probably annual mercury (Mercurialis annua)”), short for herba mercuriālis (“(probably) annual mercury”, literally “herb or plant of the god Mercury”). Mercuriālis (“pertaining to the Roman god Mercury”, adjective) is derived from Mercurius (“the Roman god Mercury”) + -ālis (suffix forming adjectives of relationship from nouns). Later adjective and noun uses may have been directly derived from Latin mercuriālis (adjective), whence Middle English mercurial (“under the astrological influence of the planet Mercury”).