stand up with
Be the principal witness at a wedding, that is, act as best man or maid or matron of honor. For example, Jane asked her sister to stand up with her.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.