The antonyms for the word "over-weight" are "underweight" and "thin". These words are used to indicate someone who is lighter in weight compared to their ideal weight or the average weight for their height and age. People who are underweight may suffer from health problems such as malnutrition, lack of energy, and weakened immune system. However, being thin doesn't necessarily imply being underweight or unhealthy. Genetics, lifestyle habits, and other health factors also play a role in determining one's weight. It's important to maintain a healthy weight range that is suitable for an individual's body type and overall health.