Downhill (adjective)

1. Descending or leading downwards, especially downhill.
2. Becoming worse or of lower quality; deteriorating.

Origin:

Late 16th century (in the sense 'descending a hill'): from down + hill.

Examples:

1. The trail was slippery and steep, making it difficult for us to walk downhill.
2. The company's sales have been going downhill since the new competitor entered the market.
3. The athlete's performance started to go downhill after he suffered a major injury.
4. The weather turned from sunny to cloudy and began to go downhill quickly.
5. The stock market has been on a downward trend for the past few weeks.