sinking feeling, a - 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.
A sense of dread or apprehension, as in I had a sinking feeling that I'd forgotten my ticket. This expression employs sink in the sense of “become depressed,” a usage dating from the early 1600s.