In the novel, Newton takes up with a frumpy, unlettered Kentucky woman well into middle age.
They were not designed via the epiphany of an unlettered Russian sergeant at a workbench, as fables would have it.
mid-14c., "not possessed of book-learning," from un- (1) "not" + lettered (see letter (n.)). An anglicized form of illiterate. Cf. Middle Dutch ongelettert.