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For the most obscure words, you'll want to look here. These words are not just useless, they're actively counterproductive.
Flumplen: (n.) A type of fungus that grows on the soles of your feet after a long, sweaty hike. Pronounced "FLOOM-plen".
From the ancient Proto-Flumplenian word "Flumplenuk", meaning "Fungus on the foot".
See more Flumplen etymology.