Boyishness is the look or way of acting that reminds people of a boy — and in this use, it is meant as a nice thing, not a complaint. A woman may have boyishness in a short haircut, a quick grin, a light walk, a kind of young energy that people find attractive. A man may have it too, if he still seems playful and young in a charming way, not stiff and grown-up. It is a noun: the quality itself. The adjective is boyish. It shows up in talk about faces, clothes, and film stars. It is not the same as being a child, and it is not the same as childish, which often sounds like you are scolding someone for acting too young.

男孩子氣（討喜的那種，不是「幼稚」）