Answer:
I believe the nonsense word "burbled" means snarled.
Step-by-step explanation:
From the description is the excerpt, the Jabberwocky seems to be a terrifying and ferocious creature. If it has eyes of flame and if it comes whiffling, we can imagine it is a scary beast. To whiffle means to produce a puffing sound. Therefore, with that information in mind, I believe the best interpretation for the word "burbled" is "snarled". It is the most probable sound to be emitted by such a ferocious-looking creature.