A psychosomatic illness or problem is one that affects both your mind and body.
When we say someone has a psychosomatic issue, we usually mean it's not just about what they can see on a doctor's chart - their symptoms also have a lot to do with how they're feeling emotionally. For example, if you get stomach problems every time you take an important exam, your mind is making your body react in a way that's not necessarily connected to the actual exam itself. Or maybe someone gets headaches when they're stressed or anxious. In both cases, it's like their mind and body are communicating with each other in a complicated dance.
(now rare) Pertaining to both the mind and the body.
