Abstruse is a word that's so obscure, you'd think it's made up, but nope! It's actually a real word that's been kicking around since the 17th century.
Etymology of Abstruse: A Brief History
Abstruse comes from the Latin word "absturcus", which roughly translates to "not to be understood" or "obscure". It's like the opposite of "obvious", but without the "-s" at the end. Confusing, huh?
See a list of related words that are almost as obscure as Abstruse
And that's the story of Abstruse. If you're still confused, don't worry, it's not your fault!