In a 2010 NYT “On Language” column, Grant Barrett traced the claim that “cellar door” is the most beautiful phrase in English back as far as
1905 1903. I posted on the phrase a few years ago ("The Romantic Side of Familiar Words"), suggesting that there was a reason why linguistic folklore fixed on that particular phrase, when you could make the same point with other pedestrian expressions like linoleum or oleomargarine:
…The undeniable charm of the story — the source of the enchantment that C. S. Lewis reported when he saw cellar door rendered as Selladore — lies the sudden falling away of the repressions imposed by orthography … to reveal what Dickens called "the romantic side of familiar things." … In the world of fantasy, that role is suggested literally in the form of a rabbit hole, a wardrobe, a brick wall at platform 9¾. Cellar door is the same kind of thing, the expression people use to illustrate how civilization and literacy put the primitive sensory experience of language at a remove from conscious experience.
But that doesn't explain why the story emerged when it did. Could it have had to do with the song "Playmates," with its line "Shout down my rain barrel, slide down my cellar door"? There's no way to know for sure, but the dates correspond, and in fact those lines had an interesting life of their own…
"Playmates" was a big hit for Philip Wingate and Henry W. Petrie in in 1894, in an age swilling in lachrymose sentimentality about childhood. The original lyrics were:
Say, say, oh playmate,
Come out and play with me
And bring your dollies three,
Climb up my apple tree.
Shout down my rain barrel,
Slide down my cellar door,
And we'll be jolly friends forevermore.
Wingate and Petrie followed it up in the same year with an even more popular sequel, “I Don’t Want to Play in Your Yard,” which containted the phrase “You’ll be sorry when you see me sliding down our cellar door." The song figures a couple of times in the 1981 Warren Beatty movie Reds, most unforgettably as sung by Peggy Lee.
In various forms, “slide down my cellar door” became a kind of catchphrase to suggest innocent friendship. In an 1896 letter to a friend, the poet Vaughan Moody wrote “Are n’t [sic] you going to speak to me again? Is my back-yard left irredeemably desolate? Have your rag dolls and your blue dishes said inexorable adieu to my cellar-door? The once melodious rain barrel answers hollow and despairing to my plaints….”
More generally, “You shan’t slide down my cellar door,” and the like were invoked to suggest childish truculence. Google Books and Newspaperarchive turn up numerous hits, which don’t tail off until the 1930s or so.
I would not let an operator that did not have a card, carry my lunch basket or slide down my cellar door: not to say give him a "square" or fix him for a ride over the road. Trans-Communicator, 1895
Commenting on a recent press dispatch Spain has refused the customary permission to the British garrison at Gibraltar to play polo and golf on Spanish territory, the Baltimore Sun says : — " This suggests the stern retaliatory methods of childhood : ' You shan't play in my back yard, you shan't slide down my cellar door.' National Review, 1898
If you see my friend Prince Krapotpin tell him I should be glad to have him holler down my rain barrel or slide down my cellar door any time. It is a hard thing to be a czar. Oak Park (IL) Argus, 1901
William Waldorf Astor seems to have carried into maturity the youthful feelings so beautifully expressed in ballads of the " you can't slide down my cellar door " school. Munsey’s magazine, 1901
And Greece has said to Roumania, "You can't slide down my cellar-door any more." Religious Telescope, 1906
I am not desirous of having him slide down my cellar door. So far as I am concerned he can stay in his own back-yard, his own puddle or whatever his habitat may be. Louisiana Conservation Review, 1940
The Abbe was gentle and courteous, not to say whimsical, and the very soul of cheerfulness, cordiality, and hospitality, but the blunt fact remained that he wouldn't play ball in my back lot or slide down my cellar door. Wine Journeys 1949
That’s the last instance of the phrase that I can find where it's used that way. The song “Playmates” enjoyed a renewed popularity when it was recorded by Kay Kyser in 1940 and of course remains popular as a children’s clapping song today. (Willie Nelson recorded a version a version a few years ago.) Notably, Kyser substituted “look down my rain barrel” for “shout down my rain barrel,” the acoustic charms of rain barrels having faded from memory along with the containers themselves, even as sloping exterior cellar doors were becoming scarce. A 1968 article in the Lima (Ohio) News began:
“Shout down my rain barrel, Slide down my cellar door, And we'll be jolly friends forever more.” Modern kids would have a hard time making friends that way, for gone are the rain barrels and outside cellar doors. Lima (Ohio) News 1968
Could the songs have been the immediate inspiration for the claim that “cellar door” is the most beautiful phrase in the English language? Well, the dates are suggestive, particularly given that the phrase was literally in air when the claim first emerged, and occasionally, no doubt, mondagreenized into something else (the way later generations often transform "rain barrel" to "rainbow"). And I think it counts for something that the perception of the phrase's beauty requires a regressive capacity, as I put it in the earlier post, to "transcend not just its semantics but its orthography, to recover the pre-alphabetic innocence that comes when we let 'the years of reading fall away,' in Auden's phrase, and attune ourselves with sonorities that are hidden from the ear behind the overlay of writing"—that is, you have assume, as the songs ask you to, a child's point of view.
But this account of the origin will be have be left speculative—unless, or course, someone digs up a pre-1894 citation for the claim, in which case the theory is toast.