For something or someone to go downhill, it means to decline and grow worse and worse.
To deteriorate; worsen.
- Gradually become worse.
1. The construction industry is showing that it’s going downhill.
2. My health has gone downhill since the accident.
3. Her immune system has gone downhill since her chemo treatment.
4. Our puppy was neutered and his appetite has gone downhill since then.
decline, decrease, degenerate, depreciate, descend, deteriorate, diminish, drop, dwindle, fade, fail, fall, flag, lower, reduce, sag, shrink, sink, suffer, weaken, worsen