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out_of_the_blue
like a bolt out of the blue
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unanticipated
[ˌənæ'ntɪsəˌpeɪtɪd]
"unanticipated and disconcerting lines of development" - H.W.Glidden
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unforeseen
[ˌənfɔ'rsin]
unforeseen circumstances
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unlooked-for
a virtue unlooked-for in people so full of energy