not anticipated; "unanticipated and disconcerting lines of development"- H W Glidden; "unforeseen circumstances"; "a virtue unlooked-for in people so full of energy"; "like a bolt out of the blue"
If something that has happened was unforeseen, it was not expected to happen or known about beforehand. Radiation may damage cells in a way that was previously unforeseen Unfortunately, due to unforeseen circumstances, this year's show has been cancelled. an unforeseen situation is one that you did not expect to happen unforeseen circumstances/events/changes etc