If you describe someone or something as ~, you mean that you cannot tell what they are going to do or how they are going to behave.
He is utterly ~.
...Britain’s notoriously ~ weather.
? predictable
ADJ
unpredictably
Monthly costs can rise or fall unpredictably.
...her husband’s unpredictably violent behavior to others.
ADV: usu ADV with v, ADV adj
unpredictability
...the unpredictability of the weather.
N-UNCOUNT: oft with poss