As Dr. Pangloss, Candide's tutor and mentor explained, whatever happens, happens for the best in the best of all possible worlds (“Tout est pour le mieux dans le meilleur des mondes possibles.”)
noun: One who is optimistic regardless of the circumstances.
adjective: Blindly or unreasonably optimistic.
After Dr. Pangloss, a philosopher and tutor in Voltaire's 1759 satire Candide. Pangloss believes that, in spite of what happens -- shipwreck, earthquake, hanging, flogging, and more -- "All is for the best in the best of all possible worlds." The name is coined from Greek panglossia (talkativeness). Earliest documented use: 1794.
"Master Pangloss taught the metaphysico-theologo-cosmolonigology. He could prove to admiration that there is no effect without a cause; and, that in this best of all possible worlds, the Baron's castle was the most magnificent of all castles, and My Lady the best of all possible baronesses...
"It is demonstrable," said he, "that things cannot be otherwise than as they are; for as all things have been created for some end, they must necessarily be created for the best end. Observe, for instance, the nose is formed for spectacles, therefore we wear spectacles. The legs are visibly designed for stockings, accordingly we wear stockings. Stones were made to be hewn and to construct castles, therefore My Lord has a magnificent castle; for the greatest baron in the province ought to be the best lodged. Swine were intended to be eaten, therefore we eat pork all the year round: and they, who assert that everything is right, do not express themselves correctly; they should say that everything is best." - Candide, Ch.1
The novel is about a series of disasters and misfortunes that befall Pangloss, Candide, and the the company that they gather, during their adventures.
The novel is a satire. It satirizes philosophy, religion, academia, the political order - basically most of the dominant institutions.
It's difficult to be or to feel panglossian these days, maybe that's why the word isn't oft encountered.
A couple of other adjectives originating from literature's characters, not used much in speech, but occasionally in writings are:
Stentorian from Homer's The Iliad. Stentor was a herald in the Greek army during the Trojan Wars, and had a loud, thundering voice. His name has been bequeathed to the adjective stentorian = loud and thundering voice.
Gargantuan from Rabelais' The Life of Gargantua and of Pantagruel, a bawdy 16th century work with scatological references. Gargantua, in Rabelais' novel, is born calling for ale, and with an erection a yard long.
Hmmm. I shall be careful how I use the latter word - if I ever do use it!
On a different note there's Pickwickian
1. Marked by generosity, naivete, or innocence.
2. Not intended to be taken in a literal sense.
After Samuel Pickwick, a character in the novel Pickwick Papers (serialized 1836-1837) by Charles Dickens. Mr Pickwick is known for his simplicity and kindness. In the novel Mr. Pickwick and Mr. Blotton call each other names and it appears later that they were using the offensive words only in a Pickwickian sense and had the highest regard for each other.
Another term that arose from the book is Pickwickian syndrome, which refers to a combination of interlinked symptoms such as extreme obesity, shallow breathing, tiredness, sleepiness, etc. The character with these symptoms was not Mr. Pickwick, but Fat Joe, so the term is really coined after the book's title. The medical term for the condition is obesity-hypoventilation syndrome.
From wordsmith.org again
Lots more examples of the same type of word, obscure and otherwise, at Wiki
List of eponymous adjectives in English