all of a sudden, without warning; unexpectedly; suddenly.Also on a sudden.
Origin of sudden
1250–1300;Middle Englishsodain (adj. and adv.) < Middle French < Latinsubitāneus going or coming stealthily, equivalent to subit(us) sudden, taking by surprise (see subito) + -āneus composite adj. suffix, equivalent to -ān(us) -an + -eus-eous
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.
late 13c., perhaps via Anglo-French sodein, from Old French subdain "immediate, sudden," from Vulgar Latin *subitanus, variant of Latin subitaneus "sudden," from subitus "come or go up stealthily," from sub "up to" + ire "come, go." Phrase all of a sudden first attested 1680s, earlier of a sudayn (1590s), upon the soden (1550s). Sudden death, tie-breakers in sports, first recorded 1927; earlier in reference to coin tosses (1834).
Entirely without warning, abruptly, as in All of a sudden the lights went out. In Shakespeare's day the common phrase was of a sudden, the word all being added in the late 1600s. Also see all at once, def. 2.