The phrase round Robin Hood’s barn means by a circuitous or tortuous route, and, figuratively, in a lengthy or roundabout way—synonym: (all) round the houses.

This phrase alludes to Robin Hood, a legendary English outlaw of the reign (1189-99) of Richard I (1157-99); according to tradition, Robin Hood lived in Sherwood Forest, in Nottinghamshire, and robbed the rich to give to the poor.

According to Robert Allen in Dictionary of English Phrases (Penguin Books, 2008), this phrase refers to a real barn:

His barn, where he stored all his booty, was presumably an out-of-the-way place, so that travelling round it involved a considerable detour.

But, in fact, the phrase alludes to a metaphorical barn, as Alfred Cocks explained in A third Contribution towards a Bucks Vocabulary, published in Records of Buckinghamshire, or, Papers and Notes on the History, Antiquities, and Architecture of the County, together with the Proceedings of the Architectural and Archaeological Society for the County of Buckingham (Volume 9 – Aylesbury, Buckinghamshire, 1909):

Robin Hood’s Barn, “all round Robin Hood’s barn,” = in all directions, everywhere, on all sides. “I’ve looked all round Robin Hood’s barn, and I can’t find him.” The considerable stretch of country which contained Robin Hood’s provisions (deer, etc.) might metaphorically be called his “barn,” and so the phrase be applied to any large space.

The earliest known use of Robin Hood’s barn is from a letter written in 1797 by the American book agent and author Mason Locke Weems (1759-1825)—I could not determine whether he used the phrase literally or figuratively, as I could not consult the letter:

[as quoted in the Oxford English Dictionary (3^rd edition, 2010)]
I can sell them abundantly fast without the trouble of going round Robin Hood’s barn.

The earliest instance of the phrase that I have found is from the Baltimore Patriot and Mercantile Advertiser (Baltimore, Maryland) of Friday 14^th July 1820—round Robin Hood’s barn is used figuratively:

The account of Colonel Duane’s offer to enlist in support of the administration for pay and rations, seems to nettle that worthy gentleman not a little. He has resorted to a curious mode of doing away the effect of the charge without denying it. He writes a letter to a friend in Harrisburg, which he procures to be printed in the Harrisburg Chronicle; and then extracts it into his own paper. This letter contains a statement of a conversation with himself; and “Colonel Duane” is the “little hero” of the tale. He alludes to documents, without mercy, and hints at evidence without measure. Pshaw, Colonel, where is the use of this “round Robin Hood’s barn” management? Speak out, yea or nay; guilty or not guilty. Do not, for pity’s sake, sicken the public with pompous, tho’ roundabout statements of letters, which you intend to write to the President, the Secretary of State, &c. &c. “The sovereignst thing on earth is parmeceti for an inward bruise.”¹

¹ This is a quotation from The history of Henrie the fourth [Part 1] (Quarto 1 – London, 1598), by the English poet and playwright William Shakespeare (1564-1616)—Hotspur describes the courtier who demanded the prisoners on the king’s behalf:

                                    He made me mad
To see him shine so briske, and smell so sweet,
And talke so like a waiting gentlewoman,
Of guns, and drums, and wounds, God saue the mark:
And telling me the soueraignest thing on earth
Was Parmacitie², for an inward bruise.

² parmacitie: a variant of spermaceti, denoting a white waxy substance produced by the sperm whale

Although first attested—and chiefly used—in American English, round Robin Hood’s barn was in dialectal use in England in the 19^th century, if not earlier. It was recorded not only in the above-quoted 1909 Contribution towards a Bucks Vocabulary, but also in The English dialect dictionary: being the complete vocabulary of all dialect words still in use, or known to have been in use during the last two hundred years (Oxford, 1905), edited by the English philologist Joseph Wright (1855-1930):

Robin Hood’s barn, in phr. to go round by Robin Hood’s barn, to go a roundabout way, to go the farthest way […].
Cambridgeshire: “No, you come with me this way; it’s the nearest. I’m not going round by Robin Hood’s barn.”
West Middlesex: “I didn’t know the way, so I went all round Robin Hood’s barn before I got there.” [from an unprinted collection by W. P. Merrick]

The earliest British-English use of Robin Hood’s barn that I have found is from a letter to the Editor by one Henry Dowell Griffiths, declaring himself “a foe to every species of oppression”, published in The Northern Star, and Leeds General Advertiser (Leeds, Yorkshire) of Saturday 21^st August 1841—“as the saying is” indicates that the phrase was already well established:

Sir,—Permit me, through the medium of your journal, to call the attention of the inhabitants of Paddington and Marylebone to the stoppage of a right of way, at the end of Westbourne Grove, a new road running in the direction of the Western Railway terminus to Notting-hill-fields, and which, until lately, (when the “improvements” were extended a field’s length,) was free to every class of pedestrians who had occasion to pass that way. I have spoken to Mr. Jones, the builder, of Westminster, on whose premises (last house in the Grove) I considered an opening ought to be made, but he objects to it for several reasons—the chiefest (and most frivolous) of which is, that it would defeat his intentions of keeping the road “select.” “If,” said he, “a thoroughfare were to be made, the poor people (mark this, ye labouring classes) would pass this way; and (mark, again) they do destroy everything so.” I told him I knew too much of the industrious classes to believe that. But even admitting his assertion to be true, I believed they (the industrious classes) paid for everything, and made the rich what they are. “Yes,” said he, “but I should not like them to pass this way; you know one likes to keep the place as select as one can.” And so the poor are to go “all round Robin Hood’s Barn,” as the saying is, because one man has taken it into his bead to make what should be a most necessary thoroughfare “select.”

A relatively large number of individuals known as Robert Hood, or Robin Hood and variants, are recorded in England from at least the early 13^th century—Robin being one of the pet forms of Robert³. This probably explains why a field, named after a barn, itself named after one Robin Hood, is mentioned in the following announcement, published in The Sussex Advertiser: Or, Lewes and Brighthelmston Journal (Lewes, Sussex) of Monday 10^th November 1823:

                                                  TO BE SOLD BY AUCTION,
                                                       By THOMAS & SON,
At the Swan Inn, Hastings, on Saturday, the 15^th day of November, 1823, at four o’clock in the afternoon. Part of the Estate of the late Mr. Christopher Hoad, of Icklesham, in the following Lots;—
                                                                 Lot 1.
A GOOD FARM HOUSE, barn, stable, yards, and lodges, with a Cottage, and Six Pieces of LAND, called The Grove, Barnfield, Five Acre Field, Eight Acre Field, Cinder Bank Field, and the Hilly Field, containing together 28A. 1R. 24P.
                                                                 Lot 2.
THREE PIECES of LAND, called by the several names of the Foxearth, Stone Croft, and Three Acre Field, containing together 8A. 1R. 14P.
                                                                 Lot 3.
TWO PIECES of LAND, called Beaney’s, and Robin Hood’s Barn Field, containing 7A. 3R. 8P.
[&c.]

³ On pet forms of Robert, cf. also:
– The usual explanation of ‘Hobson’s choice’ is fallacious.
– ‘hobby’, originally a diminutive of ‘Hob’, pet form of ‘Robert’
– meaning, origin & history of ‘bob’s your uncle’
– origin of ‘bobby’ and ‘peeler’ (‘police officer’)