"Luminous" (early 15th century) originates from the Latin lūmen (genitive lūminis), meaning light, with the suffix -ōsus (-ous), meaning "full of" or "abounding in".  It entered English via Old/Anglo-French lumineux or directly from Latin, ultimately rooting in the Proto-Indo-European (PIE) root leuk- ("light, brightness").  It originally meant "Full of light, shiny, or brilliant", but by the late 15th century, it was also used to describe clarity or understanding, similar to "lucid".  It shares roots with words like lumen, luminary, and illuminate. 

According to the Oxford English Dictionary, the earliest known use of "luminous" in English dates to approximately 1450.  The word appears as lumynouse in the context of describing a "virgine fulle of splendour" in the Mirour Saluacioun, a Middle English translation of the Speculum Humanae Salvationis.  Luminous was frequently used to describe a shining or brilliant appearance before later adopting figurative meanings relating to clarity of intellect or writing.  

Luminous writing refers to prose or poetry that is exceptionally clear, brilliant, radiant, and enlightening.  Metaphorically, it means writing that "shines" due to its beauty, intellectual clarity, or profound insight. It often describes literature that is vivid, easily understood, and intellectually stimulating.  

Such a lovely word. Must try write with this quality… if only