"Not a dicky bird"
-- Ludo Bagman, when asked if he'd heard anything from Bertha Jorkins (GF7)
Not a word.
Dicky bird is rhyming slang for "word". This is actually one of the simpler examples of rhyming slang, in which a rhyming word or phrase is substituted for the given word. In more involved examples, the actual rhyming syllable is dropped. (An example of that sort of thing would be "loaf" as a slang term for "head", where "loaf" is short for "loaf of bread".)