adverb
1. : all at the same time
seemed happy and sad all at once
2. : with great suddenness : suddenly and unexpectedly
all at once there was a clatter of dishes
adverb
1. : all at the same time
seemed happy and sad all at once
2. : with great suddenness : suddenly and unexpectedly
all at once there was a clatter of dishes
Webster's New International English Dictionary. Новый международный словарь английского языка Webster. 2012