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heart in one's mouth, have one's

Be extremely frightened or anxious, as in When the plane was about to take off, my heart was in my mouth. This usage alludes to the heart beating so violently that it appears to leap upward. [Mid-1500s]

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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.