heart sinks, 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.
One's courage or hope fails; one is very disappointed or dejected. For example, An hour before the picnic I heard thunder and my heart sank. This expression was first recorded in 1605 but was preceded in the 15th century by one's heart is at one's heels or in one's hose or in one's shoes. The present (and only surviving) usage was first recorded in 1605.