If you describe someone or something as unpredictable, you mean that you cannot tell what they are going to do or how they are going to behave. He is utterly unpredictable. Britain's notoriously unpredictable weather. predictable + unpredictably un·pre·dict·ably Monthly costs can rise or fall unpredictably. her husband's unpredictably violent behavior to others. + unpredictability un·pre·dict·abil·ity the unpredictability of the weather