By looking around the internet, it seems like a lot of people have not heard it before Harry Potter. According to dicitonary.com it has the meaning of: a young, well-educated person who has a professional career and aspires to a higher social and economic status Here is the source: https://www.dictionary.com/browse/yumpy It also says that the origin of this word was from the pharse: y(oung) u(pwardly) m(obile) p(rofessional) (or p(erson)

This appears to be either a made up word or exclusive to a British Public School https://www.reddit.com/r/harrypotter/comments/2krt7g/movie_specific_vocabulary_question/ Gives a good explanation