As a verb, it means to employ nonsense to those ends. In other words, to BS someone.
The slang term is American, and over two centuries old. It got its meaning from two sources:
- The false claims by thousands of Irish Americans "to have kissed the Blarney Stone," when in fact they'd never set foot in County Cork.
- Lady Blarney, a smooth-talking liar who appears in Oliver Goldsmith's 1766 novel The Vicar of Wakefield.
But some blarney is ominous: it's meant to fool.
The term for that form of blarney is trumpery.
Trumpery is a Middle English word that refers to anything that's attractive, but of no real value. "Truthful hyperbole" fits the category.
Savor blarney, but beware of trumpery.
And, no blarney, Happy St. Patrick's Day!