Meaning of “downhill” - Learner’s Dictionary

downhill

adverb uk us /ˌdaʊnˈhɪl/

towards the bottom of a hill or slope:

It's so much easier cycling downhill.
go downhill

to gradually become worse:

After his wife died, his health started to go downhill.

(Definition of “downhill adverb” from the Cambridge Learner’s Dictionary © Cambridge University Press)