1796, from Blarney Stone (which is said to make a persuasive flatterer of any who kiss it), in a castle near Cork, Ireland. As Bartlett explains it, the reason is the difficulty of the feat of kissing the stone where it sits high up in the battlement: "to have ascended it, was proof of perseverence, courage, and agility, whereof many are supposed to claim the honor who never achieved the adventure." So to have kissed the Blarney Stone came to mean "to tell wonderful tales" ["Dictionary of Americanisms," 1848]. The word reached wide currency through Lady Blarney, the smooth-talking flatterer in Goldsmith's "Vicar of Wakefield" (1766). As a verb from 1803.
Smooth, flattering talk, often nonsensical or deceptive. Based on an Irish legend that those who kiss the Blarney Stone will become skilled in flattery.