Ruddy is used to describe something that is reddish — like the color of red hair, tomatoes, or a friend's cheeks on a cold winter's day.
Ruddy is commonly used to describe someone's complexion. In this case, it describes a healthy, reddish glow. If you're normally pale, your skin might have a ruddy tone from working outside. You may hear someone use ruddy as a negative adjective, as well. In Britain it is a substitute for words like bloody or darn: "The ruddy door is stuck again. I'm going to take an axe to it!"