( BrE , informal ) to say, hear, etc. nothing :
He won't say a dicky bird, but we think he knows who did it.
NOT SAY, HEAR, ETC. A DICKY BIRD
Meaning of NOT SAY, HEAR, ETC. A DICKY BIRD in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005