1. Happening without warning; unforeseen: a sudden storm.
2. Happening or done without delay; hasty or immediate: a sudden decision.
3. Characterized by sharp change in elevation; precipitous: a sudden drop in the ocean floor.
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 the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.