The two characters combine literally to mean “insane person,” directly corresponding to the English terms “madman” or “lunatic.”
In Chinese culture, mental health has traditionally been a subject of significant stigma. Because of this, using the term 疯子 to describe someone with an actual mental illness is extremely insensitive and derogatory, far more so than its English equivalents might be in a casual context. It implies a complete loss of reason and social function. However, this very strength makes it a common word in everyday speech for exaggeration. The Western concept of “being crazy” for a passion (e.g., “He's crazy about football”) has a direct parallel in the hyperbolic use of 疯子. You might call a friend who pulls an all-nighter for a project a 疯子, meaning they are acting with an intensity that seems to defy normal reason. This is similar to calling someone a “maniac” or “absolute madman” in English, but the underlying severity of the literal meaning gives it a bit more punch.
The connotation of 疯子 is almost entirely dependent on the context and the relationship between the speakers. It can range from a vicious insult to a term of endearment.
When used in an argument or to describe someone you dislike, 疯子 is a strong, direct, and offensive insult. It implies the person is irrational, out of control, and not worth reasoning with.
This is its most common usage in modern, informal Chinese. It's used to describe behavior that is extreme, obsessive, or defies social norms. It's not meant literally but serves to emphasize the intensity of the action.
Between close friends, calling someone a 疯子 can be a way to express awe or disbelief at their daring or unconventional actions. The tone of voice is key. It's similar to a friend in English saying, “You are insane for doing that!” after you tell them you're going skydiving.