No language can ever have too many words with which its speakers may deride an assertion as hogwash, codswallop, baloney, poppycock, twaddle, cobblers, bosh, tosh or stuff and nonsense.
Tilly-vally is a member of this set, these days usefully obscure. It also has the imprimatur of having been employed by our greatest playwright, though in an older spelling:
Hostess Quickly: Tilly-fally, Sir John, never tell me; your ancient swaggerer comes not to my doors.
Henry IV, Part Two, by William Shakespeare, 1597-8. He uses it again in Twelfth Night.
It’s fairly common in writings down to the latter part of the nineteenth century:
“So please your Ladyship, we do not think of marrying her as yet,” returned Susan, in consternation. “Tilly vally, Susan Talbot, tell me not such folly as that. Why, the maid is over seventeen at the very least!”
Unknown to History, by Charlotte M Yonge, 1882.
In the century since then we have come to prefer more boisterous epithets with which to express our disapprobation, letting it fall away with other derogatory expressions, such as the imitative pshaw!.
Various forms are known, such as tillie-vallie, tilley-valley and tillie-wallie as well as tilly-fally. The source is quite remarkably obscure. Some older dictionaries insist it’s Scots in origin. Other authorities have claimed it was a hunting phrase borrowed from the French (presumably connected with tally-ho!) or that it was a mere minor variation on fiddle-faddle.
Sir Walter Scott had a character suggest it derives from the Latin titivillitium, a trifle or a trivial item of gossip, which would make it a relative of titivil. Modern etymologists wonder about a connection with dilly-dally; Anatoly Liberman commented in his Oxford Etymologist blog in 2007 that “The sound group dil, along with till-, suggests something frivolous. It alludes to meandering and useless work.”