An absent-minded person forgets little things — keys, a bag, why they walked into a room — not because they are foolish, but because their mind is somewhere else. The “absent” here is the mind: it has gone off to a thought, a worry, a plan, and the hands keep moving without it. It is a kind word, most of the time. We use it for the professor in a story who wears two different socks, or for a parent who puts the milk in the cupboard. Though actually, it is not the same as being careless on purpose, and it is not the same as a serious memory illness. Absent-minded is everyday and often fond. If someone cannot make new memories at all, that is a different problem, and we do not use this word for that. You can also do an absent-minded thing: an absent-minded smile, an absent-minded nod — you did it without really being in the moment.