An afterthought is something you add, say, or do later, because it was not in the plan from the start. You might finish a letter and then add one more line as an afterthought. A company might add a safety part as an afterthought — meaning they did not really design it in; they stuck it on later, and it often shows. The word can sound a bit unkind: if a person feels like an afterthought, they feel they were not important enough to be included at first. It can also be light: Oh — and bring the keys, as an afterthought. Though actually, it sits opposite the idea of thinking ahead. The word is after- plus thought, and it was probably shaped to match forethought.