What is "heartsink"

Usage examples of "heartsink".

All my nearest and dearest know who Barmy Brian is, just as they know the names of every one of my heartsink patients, and I have told David that if either of my children attach an adjective, any adjective, to his first name in his presence, then he or she will not be eating en famille for a statutory minimum two years, including Christmas Day and birthdays.