1
general::
Deteriorate, worsen, as in Ever since the recession began, the business has been going downhill. The figurative use of downhill for "decline" dates from the mid-1800s. Also see DOWNHILL ALL THE WAY.
American Heritage Idioms
2
general::
Phrase(s): go downhill
[for something] to decline and grow worse and worse. • This industry is going downhill. We lose money every year. • As one gets older, one’s health tends to go downhill.
McGrawhill's American Idioms And Phrasal Verbs