The crossword solution for the term ONE GOING DOWNHILL FAST is SKIER, which is a five-letter word. A skier is someone who participates in the sport of skiing, which involves sliding down a snow-covered slope on skis. Skiing is a popular winter activity and can be done recreationally or competitively. Going downhill fast is a common phrase used to describe someone who is in a rapid decline, and in this case, it is referring to a skier going downhill quickly.