In the United States, 4-F is an old army label from the draft — the time when the country called young men into the military. Doctors and officers put each man in a group. 1-A meant “fit; we may take you.” 4-F meant the opposite: this person may not join, because of the body, the mind, or the person’s record. “Physical” could mean a heart problem, very bad eyes, or a body the army did not want. “Mental” could mean illness of the mind, or a test score the army called too low. “Moral” is the messy one. It was about the kind of life the army judged — a crime record, for example. In older times the army also used this box for things many people today would not accept as fair, such as being gay. So the tidy word “moral” hid a lot of judging. The United States has not used a real draft since the 1970s, but the name still lives in stories, films, and family talk: “He was 4-F, so he stayed home.” Younger Americans may know it only from history class. It is not daily chat in Taiwan, but you will meet it in US books and films about those wars.

美軍「4-F」免役體位。台灣說「免役」或「體位不合格」，是另一套制度。