flat on one's back
Sick in bed. For example, The flu has put her flat on her back.
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Helpless, without recourse, defeated, as in I wish I could help but the recession has put me flat on my back.
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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.