If something or someone is moving downhill or is downhill, they are moving down a slope or are located towards the bottom of a hill. He headed downhill towards the river uphill Downhill is also an adjective. downhill ski runs
a ski race down a trail the downward slope of a hill toward the bottom of a hill; "running downhill, he gained a lot of speed" toward a lower or inferior state; "your performance has been going downhill for a long time now
If you say that something is going downhill, you mean that it is becoming worse or less successful. Since I started to work longer hours things have gone steadily downhill