These old clothes were treated as castoffs and thrown away.

castoffs 是名词 castoff 的复数形式。castoff 由动词短语 cast off（丢弃、抛弃）演变而来，指"被丢弃的物品"或"废弃物"。cast- 表示"投掷、抛弃"，off 表示"离开、脱离"，组合起来表示"丢弃的东西"。