The word splendid emerged in the 1620s as a shortening of the existing adjective splendidous, and that was taken directly from Latin splendidus, which had a lot of different definitions, including "bright", "glittering", "distinguished", "fine", and "noble", among others - kind of a catch-all for describing excellent things. Splendidus traces to the verb splendere ("to be bright"), which is also the source of splendor, through Anglo-French esplendour, and resplendent, with the addition of the re- prefix (which in this context is intensive and not the "again" definition you're probably most familiar with). Finally, splendere is reconstructed back to the Proto-Indo-European root splnd, which had to do with things manifesting in general. According to Google Ngrams, literary usage of splendid has been declining since a peak in the 1830s, which is sad, because it's such a splendid word.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.