'all at once' definitions:

Definition of 'all at once'

From: WordNet
adverb
All at the same time; "Let's say `Yes!' all at once" [syn: all together, all at once]
adverb
Without warning; "all at once, he started shouting" [syn: all of a sudden, all at once]

Synonyms of 'all at once'

From: Moby Thesaurus