Oxford dictionary defines wouldn't be seen (or caught) dead as meaning:
informal Used to express strong dislike for a particular thing.
I am wondering about both the conceptual and textual origins of this phrase.
As far as I can tell, the conceptual origin is that even if the subject were dead, and thus had no control over what happened to them, they still would not do something. This explanation is also offered here:
It means, of course, "Even if I were dead I wouldn't…" The illogic behind this is part of the charm—it demonstrates the extremity of the feeling being expressed, as well as lending a touch of the picturesque.
An alternative conceptual explanation is offered by @cobaltduck:
Suppose I was somehow convinced to do X, but while doing it I had a heart attack or was struck by lightning, or… When my body is discovered, the evidence around me makes it clear I had been doing X at the time I died. I won't hear the derisive remarks, won't suffer the embarrassment, won't feel any of the repercussions. Yet that is how people will remember me, "Cobaltduck died while Xing." I do not want that to happen, therefore I will not X.
The idea is that the subject won't do something because they fear their corpse will be found in the act of doing it, thus shaming them posthumously.
Alternately, and less plausibly, some people conjecture that the phrase is linked to the phrase dead to rights (for example, here).
There are thus multiple viable explanations of the phrase. But is there any evidence that one should be favored over the other?
I am also curious to know about first textual occurrences of the idiom and its variants. For example, did "wouldn't be seen dead" take off before "wouldn't be caught dead"?
Lastly, I am curious to know if this is chiefly an American or British idiom?
Given that there seems to be no definitive answers elsewhere on the internet, I figured I'd ask the questions here.
Best Answer
A possible nautical origin
The (ever-popular) nautical origin should come, and go, first, not least because it also seems to be the earliest of candidates approximating the sense of the expression '[somebody would not be caught|seen|found] dead [doing something|being somewhere]' (hereafter WNCD):
(From A memoir of Capt. W. T. Bate, R. N., Rev. John Baillie, 1859. Emphasis mine.)
Here the sense is "without forward motion", "dead in the water", "unable to steer". While any sailor would loathe being so "caught dead", and the possible nautical origin deserves a mention, I have difficulty reconciling this 1858 use with the later sense of WNCD, not least because it expresses that something detestable has happened or might happen to somebody, while WNCD expresses the obverse, that somebody does or would detest something.
Findings
From that stopping point, my analysis becomes more tentative. Other than the negative element of the first finding, the following bulleted points are supported under the heading Evidence from press corpora, below:
Dated lexical sources
WNCD is dated to the first half of the 1900s in The American Heritage® Dictionary of Idioms by Christine Ammer:
(The American Heritage Dictionary of Idioms by Christine Ammer. S.v. "caught dead, wouldn't be." Retrieved August 17 2016 from http://idioms.thefreedictionary.com/caught+dead%2c+wouldn%27t+be.)
In OED Online, WNCD is attested from 1924-1966, and appears as
["dead, adj., n., and adv.". OED Online. June 2016. Oxford University Press. http://www.oed.com/view/Entry/47615?rskey=WsH8K8&result=1&isAdvanced=true (accessed August 17, 2016).]
Observing that both the OED and AHDICA give the 'seen dead' form of WNCD, but the OED also gives only the 'found dead' form while AHDICA also gives only the 'caught dead' form, it seems likely that 'found' is the more likely idiom for the WNCD phrase in BrE, and 'caught' the more likely idiom in AmE.
Evidence from press corpora
All images (except as noted individually) are provided via Chronicling America: Historic American Newspapers, Lib. of Congress.
Early 'found dead' attestation:
(Memphis daily appeal. (Memphis, Tenn.), 10 Sept. 1868.)
(Evening star. (Washington, D.C.), 27 April 1870.)
(The New daily appeal. volume (Carson City, Nev.), 10 Sept. 1872.)
Early 'caught dead' attestation:
(The weekly Caucasian. (Lexington, Lafayette County, Mo.), 07 Dec. 1872.)
(The Ottawa free trader. (Ottawa, Ill.), 07 Feb. 1874.)
(Alexandria gazette. (Alexandria, D.C.), 17 July 1874.)
'Caught dead' as abridgement of 'caught dead to rights' (image from Google Books):
(From "The Autobiography of Joseph Jefferson", The Century, Volume 39, 1890.)
Early 'seen dead' attestation:
(The Kalispell bee. (Kalispell, Mont.), 11 July 1901.)
(The San Francisco call. (San Francisco [Calif.]), 11 Feb. 1913.)