na-adjective, noun which may take the genitive case particle 'no', noun
sudden; unexpected; unforeseen
Describes something happening without warning or anticipation. Often used in adverbial phrases like 不意に (suddenly) or in compounds such as 不意打ち (surprise attack).
不意の来客に驚いた。
I was surprised by the unexpected visitor.
彼は不意に立ち上がった。
He suddenly stood up.
突然 is a more common and direct word for 'suddenly', while 不意 emphasizes the lack of anticipation or forewarning, often with a nuance of being caught off guard.
意外 means 'unexpected' in the sense of being contrary to expectations or surprising, whereas 不意 focuses on the suddenness and lack of prior notice.
Compound of 不 (fu, 'not') and 意 (i, 'mind, expectation'), literally meaning 'not in mind' or 'unexpected'. The reading ふい is the on'yomi of both characters.