heart in one's mouth, have one's - Phrases/Idioms
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.
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]