(n.) a compulsive liar, said to ‘lie nineteen times more than the Devil himself’
In Scots English and Scots folklore, waghorn is an old epithet for the Devil—and, more broadly, a nickname for any equally (or even more) untrustworthy character.
Unsurprisingly, the name waghorn alludes to the Devil’s goat-like horns (though the image of them ‘wagging’ is, admittedly, a more puzzling one). As a nickname for the Devil, waghorn has been in use in Scots since at least the early eighteenth century, typically as a euphemistic name called upon when you don’t want to mention the Devil’s name. (Scots folklore is full of titles like this, including the brilliant triumvirate of Auld Clootie, Auld Harry and Auld Sandie.)
In later use, however, waghorn established itself as a nickname for anyone as duplicitous as the Devil—and eventually even emerged as the name of a fabulous character from folklore in his own right, who is “more mendacious than the Devil” and considered “the greatest of all liars”, according to the Scottish National Dictionary. It’s likely from this later development that the proverb claiming a waghorn is someone who lies “nineteen times more than the Devil” developed in the later 1700s.
A fabulous personage, who, being a liar nineteen times (or, according to others, four and twenty times) greater than the devil, was crowned King of Liars. Hence, extravagant liars are said to be “as ill as Waghorn,” or “waur [worse] than Waghorn.”
John Jamieson, Supplement to the Etymological Dictionary of the Scottish Language (1825)
In the North of Scotland, it is a proverbial phrase to say of a great liar that “he lies like Waghorn,” or is “waur than Waghorn,” that “he is as false as Waghorn, and Waghorn was nineteen times falser than the devil.”
Charles Mackay and Allan Ramsay, A Dictionary of Lowland Scotch (1888)