Someone who's unlettered hasn't read many books or spent much time in school. Your unlettered cousin might not understand the Shakespearean quotes you like to throw into conversation.
Use the adjective unlettered to describe a person who is uneducated or ignorant. In countries where it's difficult for girls to get an education, women end up being unlettered. It's generally harder to get a good paying job when you're unlettered. Unlettered is an anglicized form of the word illiterate, and its root is the Latin litteræ, which means "literature or books," and also "learning or schooling."