Incapable of being foreseen or anticipated(adjective)
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Use "unforeseeable" in a sentence
"But what remains unforeseeable is its true begetter, its receiver."
"The result might well be to alter the balance of the Constitution in unforeseeable and perilous ways."
"But they can hardly be called unforeseeable, and therein lies the nub of the critical questions this incident will raise for regulators everywhere: To what extent should nuclear safety regulation take account of all foreseeable contingencies, and should new technologies be required to apply to pre-existing facilities that were built to the standard of the industry at the time of construction?"