expression, noun-modifying (irregular)
Prenominal expression used to describe events, situations, or outcomes that were not anticipated. Slightly formal or literary tone due to the classical negative せぬ.
See also: 予期
An unexpected problem occurred.
I was surprised by the unforeseen visitor.
意外 is a common na-adjective/noun meaning 'surprising, unexpected' and can stand alone as a predicate, whereas 予期せぬ only modifies a following noun.
Compound of 予期 (expectation) + classical negative auxiliary verb ぬ (not), in its prenominal form せぬ. The form せぬ is a contraction of せず (the classical negative continuative) used attributively.