word-meaning
"You know what could be in there?" said Luna eagerly, as the wall started to spin yet again.
"Something blibbering, no doubt," said Hermione under her breath …
Harry Potter and the Order of the Phoenix
I can't find the definition of the word 'blibbering' in any dictionaries. What does it mean?
The "standard" spelling is
Scary-looking thing, isn't he?
I'm not entirely sure, but I believe this kind of spelling is called "eye dialect":
Eye dialect is the use of nonstandard spelling for speech to draw attention to pronunciation. The term was coined by George Philip Krapp to refer to the literary technique of using nonstandard spelling that implies a pronunciation of the given word that is actually standard, such as wimmin for women; the spelling indicates that the character's speech overall is dialectal, foreign, or uneducated. This form of nonstandard spelling differs from others in that a difference in spelling does not indicate a difference in pronunciation of a word. That is, it is dialect to the eye rather than to the ear. It suggests that a character "would use a vulgar pronunciation if there were one" and "is at the level of ignorance where one misspells in this fashion, hence mispronounces as well".
The term is less commonly also used to refer to pronunciation spellings, that is, spellings of words that indicate that they are pronounced in a nonstandard way. For example, an author might write dat as an attempt at accurate transcription of a nonstandard pronunciation of that. The rest of this article will discuss the former definition.
(Wikipedia)
There's likely a technical point here I'm unaware of. Anyway, at the very least, the point of this spelling is to show that he speaks very differently from the others. I imagine it possibly reflects a real-world dialect, but I wouldn't know which one.
The author used the singular form of withers:
Withers: (veterinary medicine) The part of the back of a four-legged animal that is between the shoulder blades; in many species the highest point of the body and the standard place to measure the animal's height.
The meaning is "wings sprouted from the withers of each horse", or "wings sprouted from each horse's back".
Formally, withers can only be used in the plural, but I guess that the author has used her writer's license to "invent" singular wither.
Best Answer
It's a made-up word. It has no meaning.
From Muggles' Guide to Harry Potter/Magic/Blibbering Humdinger at Wikibooks:
Although it has no meaning, it might have been a play on words intended to convey the same kind of meaning as gibbering or blithering, which it sounds like a combination of. From both gibber and blither, respectively:
But that's purely speculation on my part.
Hermione's comment is mainly meant as a snide comment, referring back to her statement that no such creature exists. (But the syntax makes it sound as if blibbering is something that a humdinger does.)