In this text a blacksmith has seen a baby who he thinks is cursed due to his small size for his age. The mother says he's fine “How dare you?” she cried. “There’s nothing wrong with my baby.” She ran to the bar. She snatched little Jimmy from the cradle and held him in her arms. “He’s not quite plumb,” said the blacksmith. “He’s not on the right lines.” What's the meaning of plumb in this context, could it be related to his weight?