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.
ETYMOLOGY:
Middle English sodain, from Old French, from Vulgar Latin *subitnus, from Latin subitneus, from subitus, from past participle of subre, to approach stealthily : sub-, secretly; see sub– + re, to go; see ei- in Appendix I.
s
d
n
nus, from Latin subit
re, to approach stealthily : sub-, secretly; see sub– + 
