If someone is round-shouldered, their shoulders curve forward or slope down, so the top of the back looks a little hunched rather than square and open. It is about the line of the shoulders, often from years of sitting, carrying heavy bags, or simply how a body has settled. Though actually, people also use it as a fairly stable description of someone’s build, not only of one slouchy afternoon. It is not a doctor’s word, and it is not kind if you say it to someone’s face; you will see it more in writing, in older novels especially, as a quick sketch of a tired, shy, or worn-down person. Younger speakers might just say they slouch, or that they have rounded shoulders. Do not hear “round” as “fat and round”: the roundness is the curve of the shoulder line, not size. Next to a noun it is usually hyphenated: a round-shouldered boy.