I was reading a short story by Roald Dahl (The Butler), and I came across with the expression "colossal bore". I checked lots of dictionaries, but I couldn't really understand what it meant. I believe it is about his being a "nouveau riche" but I am not sure.
The sentence is : "In due course, Mr Cleaver came to regard himself as an expert on wine, and inevitably he turned into a colossal bore."

Can anyone please explain what it really means? I'll really appreciate it..

Thanks a lot.