Malarkey is meaningless talk, nonsense or foolishness.
It’s still known in the US and to a lesser extent in the UK and elsewhere, but where this odd-looking word comes from is decidedly uncertain. What we do know is that it began to appear in the US in the early 1920s in various spellings, such as malaky, malachy, and mullarkey. Its first known user was the cartoonist T A Dorgan, in 1922, but it only began to appear widely at the end of the decade. By 1930, Variety could pun on it: “The song is ended but the Malarkey lingers on.”
Various theories have been advanced. Eric Partridge pointed to the modern Greek word malakia but he formed a group of one. His later editor, Paul Beale, noted the London expression Madame Misharty, the personification of sales talk, exaggerated claims, and wild predictions, a name that was supposedly that of a fortune teller. But this is stretching a possible linguistic link to breaking point and, in any case, we know it started life in North America. Others point to the family name Malarkey, though who the eponymous member of the tribe might have been whose Irish-derived gift of the gab could have given rise to the name remains unknown. Jonathon Green likewise suggests a Irish origin in mullachan, a strongly-built boy or ruffian, though this, too, seems a stretch of meaning.
We’ll just have to settle for the unsatisfactory “origin unknown”.