A decision, action, or remark that is ill-considered has not been thought about carefully. Someone spoke or acted too fast, and later it looks unwise. It is close to ill-conceived, though actually they are not quite the same: ill-conceived often attacks the plan as a whole, from the first idea; ill-considered often attacks the moment of choosing — they did not stop to think. You can have an ill-considered comment in a meeting, or an ill-considered move in a game. It is a little formal. Ill- again means badly, not sick.