na-adjective, noun which may take the genitive case particle 'no', noun
Describes something that is contrary to expectations or surprising. Often used in formal or written contexts.
彼の反応は意表外だった。
His reaction was unexpected.
意表外な結果に驚いた。
I was surprised by the unexpected result.
意表 means 'surprise' or 'something unexpected', often used in the phrase 意表を突く (take by surprise). 意表外 is an adjectival noun derived from it.
Compound of 意表 (surprise, unexpectedness) and 外 (outside), literally 'outside of expectation'. The exact historical derivation is uncertain.