(n.) a flagrant, compulsive liar
A waghorn or waugram is a habitual liar—who, according to an old expression, lies even more frequently than the Devil.
Waghorn is a dialect term, recorded since the seventeenth century at least, particularly in Scots and North Country English. The Devil himself lurks in its etymology, as the notion here is simply of someone so mendacious that they take on a demonic countenance, with their metaphorical ‘horns’ seemingly ‘wagging’ as they speak their lies.
The Scottish National Dictionary explains that Waghorn was once the name of “a fabulous character, more mendacious than the Devil, [and] the greatest of all liars.” Other records and definitions elsewhere suggest the same: that this was once the name of some folkloric character, supposedly kicked out of Hell for proving himself even more fluent a liar than the Devil himself:
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 than Waghorn.
John Jamieson, A Supplement to the Etymological Dictionary of the Scottish Language (1825)
Based on these earlier tales, the name Waghorn or Waggie fell into allusive use in the 1700s, and has remained a byword for a compulsive, unstoppable liar ever since.