"Läppä" is a indeed slang word for a joke/humor. "Kuiva" means it's boring, unfunny...dry
If somebody knows the etymology how the word "läppä" became to mean joke/humor, I'd also be interested.
I know there is a older word "leipäläpi" that's directly translated as "bread hole" that mean mouth. I wonder if it's related. Joke/humor is something that comes out of your mouth.