Downhill means going down a slope. Walk downhill, downhill from here, downhill skiing.
Physically it is the easy direction, which is why it stands for anything getting easier: it is all downhill from here — a phrase that means both easier and worse, depending on the speaker, and causes genuine confusion. Go downhill means to deteriorate. Downhill skiing is the alpine racing discipline. Note that the ambiguity is not resolvable from the words alone.
1adverbdownhillGoing down a slope.
“The bike picked up speed as we rode downhill towards the village.”
2adjectivegettingGetting worse, or getting easier.
“After he lost his job, things went downhill fast.”
downhill is a direction.
Middle English
doun
English
down
English, today
downhill
Built from
From down- + hill. down comes from Middle English doun. From Middle English doun, doune (“down”), from Old English dūne (“down”), aphetic form of adūne (“down, downward”), from earlier ofdūne (“down”) literally “off the hill”, from of (“of, off of”) + dūn (“hill, mount, dune, down”). More at Etymology 2 below. For the development from directional phrases to prepositions, compare dene, dāle, whence dal. Compare also stq, del. Cognate with Scots doon (“down”).
Etymology from Wiktionary, CC BY-SA