surprise; unexpectedness
Refers to something that catches one off guard or defies expectations. Most often used in the phrase 意表を突く (to take someone by surprise).
彼の質問は意表を突いた。
His question took me by surprise.
His jokes always catch people off guard.
意表を突く作戦で相手を混乱させた。
They confused the opponent with a surprise tactic.
I was caught off guard by the sudden question.
不意 describes something sudden or unforeseen (as in 不意に or 不意打ち), whereas 意表 specifically refers to defying someone's expectations, chiefly in 意表を突く.
unexpected; surprising
Compound of 意 (mind, thought) and 表 (surface, expression). The literal sense is 'what appears on the surface of one's mind,' which developed into the idea of something unexpected that surprises one's thoughts.