I've heard a friend say "he says he was so lucky, it's like he sat his ass in a butter tub" a few times. Even though I'm from the same area (northeast USA) as the speaker, the expression wasn't familiar to me. I'm curious to learn more about its history, but can't find much.
I did find several examples in recent books:
Hollywood Monster: A Walk Down Elm Street with the Man of Your Dreams (2009):
and Dashing Through the Snow (2008):
as well as blogs, (2009, someone writing about their 85-year-old grandmother's expressions):
“Well, he just landed in a butter tub!” – (referring to Reesie, the little puppy we found this summer)
and in a much earlier newspaper example in the Milwaukee Journal (1922, a humorous story about a lucky escape from the embarrassment of a wardrobe malfunction):
The expression, or at least the sat in version, seems to be well-known in the 1922 example (though the story includes a giant tub of butter, so perhaps the audience is supposed to understand it from context). But here my Google-fu fails me and I haven't found anything earlier. Where does the expression landed in a tub of butter come from?
I'm not sure if sat or landed is more common, if cursing or mentioning the posterior is required, or if tub of butter or butter tub is the more usual phrasing. So for bonus points*, which version is the most normal or idiomatic?
*Bonus points may be even sharper and more imaginary than regular points.
Best Answer
The earliest explanation of this saying that I've been able to turn up in a Google Books search is from Marillyn Taylor Klam, My Mama Always Said...: A Book of Southernisms (1993):
This "rolling in butter" metaphor seems like a plausible source to me, since I've heard the expression "rolling in dough" used to describe possessing great wealth.
The earliest use of the phrase that I've found in a Google Books book is in Alan J. Lerner, My Fair Lady (1956, pages 45–46):
This exchange occurs after Alfie's daughter, according to Mrs. Hopkins, has "moved in with a swell"—namely, Henry Higgins. It leads into the song "With a Little Bit of Luck." No such expression occurs in G.B. Shaw's original Pygmalion (1916).
There are also two occurrences of the same idea of "tub of butter" as "lap of luxury" in Paul Hyde Bonner, The Art of Llewellyn Jones (1959, both snippets):
and:
UPDATE (4/2/2016): Early use of 'falling into a tub of butter' as an idiomatic phrase
A search of the Library of Congress's Chronicling America newspaper database turns up three instances of "falling into a tub of butter"—all by the same reporter writing for the same newspaper, all within 15 months of each other, and all in the context of baseball. From Wagner, "Home Runs Defeat Dan O'Neil's Club" in the Bridgeport [Connecticut] Evening Farmer (June 19, 1911):
From Wagner, "Senators in Bad Wreck at Steeplechase," in the Bridgeport [Connecticut] Evening Farmer (August 14, 1911):
And from Wagner, "Umpire Red Held May Go to American Association; Other Notes of Interest Regarding Those In Sporting Spotlight," in the Bridgeport [Connecticut] Evening Farmer (September 24, 1912):
Clearly Mr. Wagner views "soft as falling into a tub of butter" as a swell go-to simile; but whether he made up the expression himself or just repeated a saying then current in Bridgeport, Connecticut, is impossible to say on the basis of the skimpy evidence here. The fact that the phrase doesn't show up elsewhere in the same figurative sense, however, makes its status as a forebear of the modern phrase rather shaky, particularly since "soft" isn't the same as "lucky."
On the other hand, further searching in Google Books yields an even older instance in which falling into butter is a figurative treat. From Edward Eldridge, A California Girl (1902):
And from Guido Janes, "Marrying for Money," in Domestic Engineering (January 13, 1917):
The 1902 and 1917 instances related to matrimony are directly on point and suggest that the phrase that the OP asks about has been in use in the figurative sense of "lucky" for more than a century.