1, 2. unforeseen, unanticipated. Sudden, unexpected, abrupt describe acts, events, or conditions for which there has been no preparation or gradual approach. Sudden refers to the quickness of an occurrence, although the event may have been expected: a sudden change in the weather. Unexpected emphasizes the lack of preparedness for what occurs or appears: an unexpected crisis. Abrupt characterizes something involving a swift adjustment; the effect is often unpleasant, unfavorable, or the cause of dismay: He had an abrupt change in manner. The road came to an abrupt end.