Uphill means going up a slope. Legs burning, bicycle in the lowest gear.
It works as an adverb and an adjective alike: walking uphill, an uphill path. The figurative use is where the word earns most of its keep, and an uphill struggle or an uphill battle is one where everything is against you from the start. That phrase is now far commoner than any literal use. Its opposite, downhill, has a matching double life, since going downhill means getting steadily worse.
From up + hill.
Etymology from Wiktionary, CC BY-SA