capricious
definition
given to sudden and unaccountable changes of mood or behavior
example
Her capricious nature made it difficult for her friends to keep up with her ever-changing whims.
synonyms
ficklewhimsicalmercurial
word origin — from French 'capricieux' or from Italian 'capriccioso', which derives from 'capriccio' meaning 'a shivering, whim' or 'from 'capra' meaning 'goat' (in reference to the unpredictability of a goat's behavior).