A nod is as good as a wink
To a person who is ready to understand or undertake something, any subtle signalling of it is sufficient. The context is usually of some undertaking that is borderline illegal or of sexual innuendo.
This proverbial saying sounds as if it might be quite modern but it is in fact a 16th century phrase originating in England. The longer version of the phrase is 'a nod is as good as a wink to a blind horse'. It might seem that this is just an elaboration of the shorter version, but it appears that the 'blind horse' version was in fact the original. The earliest examples of the proverb in print all give the fuller version, for example, in the Letters of the English lawyer and writer Joseph Ritson, February 1793:
A nod, you know, is as good as a wink to a blind horse.
It seems intuitive to interpret the longer version as meaning 'neither a nod nor a wink have any purpose, both being equally pointless'. Nevertheless, the context of the early uses have it being used with the same apparent meaning as the short version, that is, 'you may nod or wink - I will take your meaning either way'.
During the 19th century the expression began to be shortened and the blind horse was left at home. Citations from that period use the form 'a nod is as good as a wink etc.', which clearly indicates that the later usage was simply a shorthand way if writing the original.
More recently, the expression has gained currency in the form of "a nod is as good as a wink to a blind bat", which Eric Idle used in his 'Nudge, nudge' sketch in Monty Python's Flying Circus. The character Idle played was a nonsense-talking fool who came out with a string of meaningless innuendos. The choice of bat was knowing, as bats are generally regarded as blind and so calling the creature a blind bat emphasized the ridiculousness of the character's gabble... and, before anyone writes in, I know that bats aren't really blind.