If someone is boyish, they look or act a bit like a boy, and the speaker usually means that in a warm, often attractive way. A man can have a boyish smile — young, open, not heavy or stern. A woman can have a boyish haircut or a boyish figure, meaning short hair or a straighter shape, in a way people find fresh or appealing. It is ordinarily a compliment, or at least fond. It does not mean the person is a child, and it does not mean they are being silly. Though actually English has a nearby word, childish, that often sounds like a complaint — “stop being childish” — while boyish almost never scolds. You will hear boyish in descriptions of faces, energy, clothes, and charm. It is ordinary English, a little literary when people write about looks.