downhill
n. place B1 Intermediate US //ˈdaʊnˈhɪɫ// down·hill
n. moving toward the bottom of a hill or slope. You can also use it to describe a situation that is getting worse.
n. towards the bottom of a slope or incline; figuratively, into a state of decline or deterioration.
The bike picked up speed as I rode downhill.
After the lead singer left the band, their popularity went downhill very quickly.
The skiers accelerated downhill, navigating the icy patches with a precision that only comes from years of professional training on the world's most difficult slopes.
From down- + hill.
Usage
When used figuratively to mean 'getting worse', it almost always follows the verb 'go'.