If something bad is self-inflicted, you did it to yourself. Nobody else caused it. It is used for real physical harm — a self-inflicted wound, a self-inflicted injury — and this is the most serious use. In news and in hospitals, 'a self-inflicted wound' very often means the person hurt themselves on purpose, sometimes to die, so people say it carefully and quietly. But the word is also used for much lighter things, and this is where students meet it most: a self-inflicted problem, a self-inflicted headache, self-inflicted stress. Here it means 'you made this trouble for yourself, so you cannot really complain'. There is a small, dry joke in it. If you stay up all night playing games and then you are tired all day, your friend can say 'that's self-inflicted' and everyone laughs a little. Two things to notice. First, the word almost always sits in front of a bad noun — pain, wound, damage, misery, problem, disaster. You do not say 'self-inflicted happiness'. Second, it does not always mean you WANTED the bad thing. Usually it means your own choice led to it. The man who lost his money by gambling has self-inflicted poverty; he did not want to be poor, but he walked there himself. It is a formal-feeling word, more common in writing, news, and reports than in easy talk with friends — though the joking use is spoken a lot.