Describing a friend as lettered is a compliment — you're remarking on her educational success and knowledge. People are especially lettered when they've read a lot of books.

Though you'd never want to describe yourself as lettered — you'd sound like a show off — it's a great way to describe a scholarly friend. Lettered people are well-educated and well-read, and are often academics, either teaching or researching their areas of interest. The origin of lettered is littera, "letter of the alphabet" or "document," and its plural form, litteræ, which means "literature or books."

Definitions of lettered

adj highly educated; having extensive information or understanding

knowing, knowledgeable, learned, well-educated, well-read
possessing an education (especially having more than average knowledge)

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