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at once

at once idiom

At the same time, as in We can't all fit into the boat at once. [First half of 1200s] Also see at one time, def. 1.
Immediately, as in Mother told the children to come inside at once. [First half of 1500s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.