sud·den (sŭdʹn)adj. 1. Happening without warning; unforeseen: a sudden storm. 2. Characterized by hastiness; abrupt or rash: a sudden decision. See Synonyms at impetuous. 3. Characterized by rapidity; quick and swift.Idiom:all of a sudden Very quickly and unexpectedly; suddenly. [Middle English sodain, from Old French, from Vulgar Latin *subitānus, from Latin subitāneus, from subitus, from past participle of subīre, to approach stealthily : sub-, secretly; see sub- + īre, to go; See ei- in Indo-European Roots.] sudʹden·ly adv.sudʹden·ness n.