fickle

Adjective

• liable to sudden unpredictable change
  • erratic behavior
    
  • fickle weather
    
  • mercurial twists of temperament
    
  • a quicksilver character, cool and willful at one moment, utterly fragile the next
    

• marked by erratic changeableness in affections or attachments
  • fickle friends
    
  • a flirt's volatile affections
    

Related words

• Synonyms: 
  erratic , mercurial , quicksilver(a) , volatile