1
general::
1. All at the same time, as in We can't get inside all at once, so please wait your turn. [Late 1300s] 2. Suddenly, unexpectedly, as in All at once the sky darkened. For a synonym, see ALL OF A SUDDEN.
American Heritage Idioms
2
general::
Phrase(s): all at once
1. Lit. all at the same time. • The entire group spoke all at once. 2. Fig. suddenly. • All at once the little girl fell out of her chair.
McGrawhill's American Idioms And Phrasal Verbs