Meaning

adjective happening without warning or in a short space of time

originFrom Middle English sodeyn, sodain, from Anglo-Norman sodein, from Old French sodain, subdain (“immediate, sudden”), from Vulgar Latin *subitānus (“sudden”), from Latin subitāneus (“sudden”), from subitus (“sudden", literally, "that which has come stealthily”), originally the past participle of subīre (“to come or go stealthily”), from sub (“under”) + īre (“go”). Doublet of subitaneous. Displaced…

Etymology from Wiktionary, CC BY-SA 3.0