apparently 54,000. Seems like it is including even fictional words though in this test (like from fiction novels). Ironically I scored higher on the expert words (18/20) than the "advanced" words (11/20)

plenty of words and phrases originate from fiction

quixotic, scrooge, shangri-la, Uncle Tom, gargantuan, kafkaesque, blurb, milquetoast

and words like cyberspace were first used in fiction

once real people use them, they stop being fictional words

The word was "Brobdingnagian", which apparently means "giant", from the book, Brobdingnag, published in the 1700s. I know all of the words you listed, even if I don't know t he books they came from, on the other hand, I've never heard anyone use "Brobdingnagian" and I've never heard of the book it came from either.

Strangely, I knew this one. Once you hear this word it refuses to leave your head.

I don't know that one, but I do know gargantuan, and pantagruelian, which come from a 17th century novel by Rabelais as well as yahoo and Lilliputian, which come from a 1726 novel by Swift.

gargantuan and yahoo are common parlance, people actually use these, in spite of them not knowing their origin. When's the last time you've seen those written down, or spoken, anywhere aside from those nearly half millennia old books? I've never seen those.

"Yahoo" and "Lilliputian" come from the same 1726 novel by Swift as "Brobdingnagian".

Kafkaesque doesn’t originate directly from fiction like your other examples any more than a word like Dickensian does.

Well it does and it doesn't. It wouldn't be a word if Franz Kafka hadn't written any fiction. Same for Dickensian.