all at once
1. Simultaneously. Don't answer all at once. One at a time, please.
2. Suddenly and without warning. All at once, Henry began feeling sick.
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.
all at once
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.
ˌall at ˈonce
suddenly: All at once it began to rain.