The American-English expression unshirted hell denotes serious trouble.
It seems to me that the expression unshirted hell combines the following:
– the image of taking off one’s shirt before getting into a fight—cf. the phrase keep your shirt on, meaning don’t lose your temper, stay calm;
– the use of the noun hell in the sense of a severe reprimand—cf. the phrase to give someone hell.
The U.S. author, columnist, journalist and presidential speechwriter William Safire (William Lewis Safir – 1929-2009) expressed the same opinion in his column On Language, published in The New York Times (New York City, New York) of Sunday 13th May 1990—he wrote the following about the origin of the expression unshirted hell:
The picture comes to mind of a person tearing his shirt off to castigate another; a second possibility is to berate someone whose shirt has been taken off, as if to receive a whipping. The first picture is more likely the source; the unshirted one is angry and ready to give hell to the clothed recipient.
It must be added, however, that according to Merriam-Webster’s Dictionary Online, in the expression unshirted hell, the adjective unshirted means naked, undisguised, plain.
The earliest occurrences of the expression unshirted hell that I have found are as follows, in chronological order:
1-: From The Chicago Republican (Chicago, Illinois) of Thursday 4th January 1866:
Gov. Brownlow went home from the capital of Tennessee to Knoxville to spend the Christmas holidays; and while there, just to keep his hand in and enjoy himself, he wrote all the leading articles of his well-known newspaper, The Knoxville Whig. We have not seen that number of the paper, but to judge from the extracts given by some of the Governor’s enemies, it must be unusually interesting and rigorous. In fact, one of the most unscrupulous of these persons describes his efforts by the curious and rather startling epithet of “unshirted hell.” And we dare say that if the views of The Whig as regards the rebels were alone taken into account, it would require a pretty severe style of language to express them with brevity. The Governor knows that class of people by much painful experience, and he has no faith in them; indeed, he agrees with President Johnson, that treason must be made odious by just punishment; though he does not adopt the more thoughtful and more merciful policy of the President, who holds to pardoning the many, and punishing only the few more eminent traitors. Brownlow would punish them all, without exception, and without much discrimination as to the practical methods of retributive justice.
2-: From The South-Western (Shreveport, Louisiana) of Wednesday 11th August 1869:
“Going In.”—The editor of the Claiborne advocate recently, in reply to some of his friends, who wished him to pitch in to the Radical party, said that the thing couldn’t be done just yet, but as soon as the watermelon season was over he intended to give them “unshirted hell.” We presume from the following, which we find in his issue of the 31st July, that he has drawn his shirt for the work:
“It is said that this new radical party now being proposed to the people, will be composed of all the decent men of the country. Well, that lets us out, and all the black population. [&c.]”
3-: From The Brooklyn Daily Eagle (Brooklyn, New York) of Tuesday 5th February 1878:
Wells simply says, in gothic phrase, that he proposes to give the Republican National Committee what he call [sic] “unshirted hell.” And “Zach” and Bill Chandler, John Sherman, Don Cameron, Dick McCormick and others are pensively musing on “What can the old man mean?” It seems that he has never been out of Louisiana at all, and the special correspondents who wrote themselves down as dullards and laggards by saying that he was in Washington and could not be found, also wrote themselves down inaccurate Columbuses by saying that he was there at all.
4-: From the Watertown Daily Times (Watertown, New York) of Tuesday 5th February 1878:
A Times New Orleans special says Wells refuses to speak, but incidentally mentioned that he would give the District Attorney and the Parish Judge of Vernon “unshirted hell.” These are the two parties in whose interest it is alleged he caused the returns to be altered.