suddenyoudaoicibaDictYouDict[sudden 词源字典]
sudden: [13] The etymological notion underlying sudden is of something approaching stealthily or without warning, so that it takes one by surprise. It comes via Anglo-Norman sudein from late Latin subitānus, an alteration of Latin subitāneus ‘sudden’. This was derived from subitus ‘sudden’, an adjectival use of the past participle of subīre ‘approach stealthily’. And subīre was a compound verb formed from the prefix sub-, used here in the sense ‘secretly’, and īre ‘go’ (source of English ambition, exit, issue, etc).
=> ambition, exit, issue[sudden etymology, sudden origin, 英语词源]
sudden (adj.)youdaoicibaDictYouDict
early 14c., sodaine, from Anglo-French sodein or directly from Old French sodain, subdain "immediate, sudden" (Modern French soudain), from Vulgar Latin *subitanus, variant of Latin subitaneus "sudden," from subitus past participle of subire "go under; occur secretly, come or go up stealthily," from sub "up to" (see sub-) + ire "come, go" (see ion). "The present spelling was not finally established till after 1700" [OED].

Noun meaning "that which us sudden, a sudden need or emergency" is from 1550s, obsolete except in phrase all of a sudden first attested 1680s, also of a sudayn (1590s), upon the soden (1550s). Sudden death, tie-breakers in sports, first recorded 1927; earlier in reference to coin tosses (1834). Related: Suddenness.