A nod is as good as a wink to a blind horse. Whether you nod or whether you wink, if a horse is blind he knows it not; and a person who will not see takes no notice of hints and signs. The common use of the phrase, however, is the contrary meaning, viz. “I twig your meaning, though you speak of what you purpose; but mum's the word.”
“A nod is as good as a wink to a blind horse; and there are certain understandings, in public as well as in private life, which it is better for all parties not to put into writing.” —The Nineteenth Century (July, 1893, p.6).
(The Land of) (See Land Of Nod.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894