If someone is scatterbrained, their thoughts will not stay in one place long enough for the practical jobs to get done. They start a task, remember something else, put the keys down “for a second,” and then cannot say where anything is. They are not slow, and they are not unkind; the mind is simply busy in too many directions at once, so the small, ordinary steps — time, bags, names, the thing in their hand — slip. People use it most about daily muddle: lost tickets, milk in the cupboard, a bag left on the bus. It is informal, and usually said with a smile, about a friend or about yourself. Said as a judgement, it can sting. Older writing sometimes puts a hyphen: scatter-brained. Though actually, people also talk of a “scatterbrained genius”: strong with ideas, clumsy with ordinary life. That picture is the same word, not a different one.