What does it mean for someone to be "speaking with a forked tongue"? I've heard it used by my boss when referring to particular customers of ours.
This expression did not originate with television. It has a much longer history, dating possibly to the 17th century. From Wikipedia:
The phrase "speaks with a forked tongue" means to say one thing and mean another or, to be hypocritical, or act in a duplicitous manner. In the longstanding tradition of many Native American tribes, "speaking with a forked tongue" has meant lying, and a person was no longer considered worthy of trust, once he had been shown to "speak with a forked tongue". This phrase was also adopted by Americans around the time of the Revolution, and may be found in abundant references from the early 19th century — often reporting on American officers who sought to convince the tribal leaders with whom they negotiated that they "spoke with a straight and not with a forked tongue" (as for example, President Andrew Jackson told the Creek Nation in 1829) According to one 1859 account, the native proverb that the "white man spoke with a forked tongue" originated as a result of the French tactic of the 1690s, in their war with the Iroquois, of inviting their enemies to attend a Peace Conference, only to be slaughtered or captured.
As you can see, Andrew Jackson used the expression in 1829, which was well before television and even radio.
This Google NGram supports the contention that the phrase has existed for a long, long time:
This is a reference to an expression used by American Indian characters in old Western movies and TV series, notably The Lone Ranger.
I've no idea if it's any more than an invention of Hollywood, but it is supposed to be an expression used by the Indians to means that someone is speaking words that can't be trusted. The analogy is with a snake — whose words obviously couldn't be trusted.
The context is usually "White Man speak with forked tongue!"
Though "speaking with a forked tongue" may be popularly associated with the speech of American Indians in TV and film westerns, the association of forked tongues with lying speech goes far back in English literature, no doubt influenced by the biblical association of the fork-tongued Serpent with the Father of Lies.
Consider Lancelot Andrewes, "Of the Sending of the Holy Ghost," in XCVI. Sermons by the Right Honourable and Reverend Father in God Lancelot Andrewes, Second Edition (1632):
The tongue is the substantive, and subject of all the rest. It is so: And GOD can send from Heaven no better thing; nor the Devill from hell no worse thing, than it. The best member, we have (saith the Prophet;) The worst member, we have (saith the Apostle:) Both, as it is employed.
The best, if it be of GOD s cleaving; if it be of His lightning with the fire of Heaven; if it be one that will sit still, if cause be. The worst, if it come from the Devils hands. For, he, as in many other, so in the sending of tongues, striveth to be like GOD; as knowing wlll, they are every way as fit instruments, to worke mischiefe by as to doe good with. There be tongues of Angels (in I Cor. 13.1) and if of good Angels, I make no doubt but of evill; and so, the Devill hath his tongues.
And he hath the art of cleaving. He shewed it in the beginning, when he made the Serpent, lingnam bisulcam, a forked tongue, to speake that, which was contrary to his knowledge and meaning, They should not die; and as hee did the Serpents, so hee can doe others.
Both Ben Jonson, in The Masque of Queens (1609) and John Milton, in Paradise Lost (1667) take note of the forked tongue in their representation of deceitful or damned beings. In a slight variant on this theme, in The Whores Rhetorick (1683), a character named Madam Creswel finds the serpent's forked tongue responsible not for outright deception but for "poysonous humours":
Madam Creswel. It was diverting to perceive the heedless (though attentive) Sheep led astray by this seducing Shepherd, and the unwary Animals intoxicated by the poysonous humours inspired from this Serpents forked Tongue.
Likewise, from Presbyterian Church of the U.S.A., "A Narrative of the Revival of Religion, in the County of Oneida" (1826):
It is evidently the work of some enemy to Presbyterians, and to religion. It is written with considerable art; but the serpent shows his forked tongue, in the language he occasionally utters.
And from Lewis Condict, "Address, Delivered by Appointment at Morristown, New-Jersey" (July 4, 1828):
The demon of discord and of party violence, should no be suffered to pollute freedom's offering, nor to profane her festive day. His cloven foot should not tread these sacred courts, nor his forked tongue utter its discordant note within these walls. His admission is but an ill omen to those high hopes—those cheering prospects, which animated the bosoms of our fathers and sustained them in the perilous contest.
All of these occurrences precede the famous instance in which U.S. President Andrew Jackson addressed the Choctaw Indians and offered them assurances that he did not speak with a forked tongue. (For a disturbing and surprisingly impartial contemporaneous account of what prompted those assurances and how they played out, see the March 1830 and August 1830 issues of The Missionary Herald.)
Such a person is double-minded and unstable in all they do.
Ye double minded; the persons addressed are not the profane men of the world, but sinners in Zion, formal professors, hypocritical persons; who speak with a double tongue to men, and who draw nigh to God with their mouths, but not with their hearts; who halt between two opinions, and are unstable in all their ways:
protected by tchrist♦ Mar 1 '15 at 19:16
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