what does the phrase "at once" mean in this sentence?

English as a Second Language English Vocabulary, Grammar and Idioms
1

what does the phrase “at once” mean in this sentence?

I wish I could be in two places at once.

2

‘At once’ here has the sense of ‘at the same time’.

3

but I can’t find the meaning of "at once "in any dictionary?

4

Really in ANY ? :wink:

thefreedictionary.com/at+once

5

.
And here are SOME MORE.
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6

thanks a lot!