My regular readers will likely have noticed that I thoroughly enjoy chasing down the truth behind things which are commonly accepted as facts. I am fortunate to have the gift of being able to spot cracks in arguments as well as glaciers.
I am currently heavily engaged in a legal matter concerning a witness in court who, shall we say, seems to have been somewhat uninterested in assisting the court in its determination of the true facts.
There is a phrase about not telling the truth, not now so common as when I was a child, but still in frequent use: "he speaks with a forked tongue".
It is often suggested that the idea of a forked tongue comes from the snake and the Garden of Eden story. I suggest that it does not.
There is an expression in the French language: "Ma langue a fourché". It appears that this reference to a 'slip of the tongue' is a pun. The French term 'ma langue' can mean either 'my native language', 'my use of language' or 'the strong, flexible muscle that is anchored to the floor of my mouth'. The term 'fourcher' applies to forking or bifurcation. The phrase: "Ma langue a fourché", as used by speakers of French, means: "I have used one word in place of another".
I suggest that the phrase "Ma langue a fourché" was at some time in the 1700s wrongly translated into the English as pertaining to, not a mere substitution of words, but a literal forking of the tongue. The association of a forked tongue with the snake-as-symbol would have soon anchored the phrase to its English meaning, the French origins having been long since forgotten.
That, then is my suggested etymology for any phrase which expresses a lie as "speaking with a forked tongue".