(one's) heart sinks into (one's) boots

One becomes suddenly disappointed, disheartened, or despondent because of an undesirable outcome or turn of events. His heart sank into his boots when he realized that she wasn't going to accept his marriage proposal. It's horrible having to deny people entry visas and watch their hearts sink into their boots, but it's just a part of the job. My heart sank into my boots when I arrived at the scene of the accident. How could anyone have survived?

your heart sinks into your boots

used to express a feeling of sudden sadness or dismay.

This idiom has given rise to the adjective heartsink , used in the medical profession to describe a patient who causes their medical practitioner to experience such a feeling, usually as a result of making frequent visits to the surgery to complain of persistent but unidentifiable ailments.