In Britain -- where the term has a long history -- "deal" was introduced with the importation of sawn boards -- usually of fir or pine -- from a German part of the European continent. From the beginning "deal" was associated with these kinds of wood.
Its first meaning is evidently,
"A slice sawn from a log of timber (now always of fir or pine), and usually understood to be more than seven inches wide, and not more than three thick; a plank or board of pine or fir-wood",
but as shown by the Oxford English Dictionary, historically, as this term relates to topics in the timber trade, numerous variations in the meaning of deal, have crept in.
In the timber trade, specific variations, geographically, are:
in Great Britain, a deal is understood to be 9 inches wide, not more than 3 inches thick, and at least 6 feet long. If shorter, it is a deal-end; if not more than 7 inches wide, it is a BATTEN; while
in North America, the standard deal (to which other sizes are reduced in computation) is 12 feet long, 11 inches wide, and 2 inches thick. By carpenters, deal of half this thickness (1 inches) is called whole deal; of half the latter (inch) slit deal.
John Ramsay McCulloch and Henry Vethake, A Dictionary, Practical, Theoretical, and Historical, of Commerce ... ,London, Longmans, Green, 1852, and 1869, volume 2, page 316:
Scotch fir is the most durable of the pine specie». It was the opinion of the celebrated Mr. Brindley, " that red Riga deal, or pine wood, would endure as ...
Here is the definition in the 1877 Knight's American Mechanical Dictionary
Deal A plank 12 feet long, 11 inches wide, anil 2h inches thick. Deals are sawed of other sizes, but are reduced to that cubic dimension in computing them.
Practice may differ in different countries. The above is the Ottawa rule. In England, lumber not exceeding 3 inches in thickness and 9 inches wide.
Source: Edward H Knight, Knight's American Mechanical Dictionary, a description of tools, instruments, machines, processes and engineering ; history of inventions... New York : Hurd and Houghton, 1877, page 680.
The Oxford English Dictionary shows countless historical uses of deal, including one that traces back at least to 1402, as displayed in Charles Frost, Notices relative to the early history of the town and port of Hull London : Nicholas, 1827, page 18.
from the Hogg's Weekly Instructor we get this two-step reference:
1843 Charles Holtzapffel, Descriptive Catalogue of the Woods commonly employed in this Country for the Mechanical and Ornamental Arts ,
It is in the character of timber trees that the Pines rank of most importance in an economical point of view; and in the little known work of Holtzapffel, on turnery -- woods, we find the following curt but valuable notes on the more important species, from the able pen of Dr Royle, of the East India House, who furnished important notes to the work:
The pines and firs being so numerous, and the timbers of many being known in commerce by such avariety of names, it is difficult to ascertain the trees which yield them. The Pinus sylvestris, however, called the Wild pine, or Scotch fir, yields the red deal of Riga, called yellow deal in London; Abies excelsa, or Norway spruce fir, yields white deal; Abies picea, or Silver fir, has whitish wood, much used for flooring; Larix europsea is the Larch common in the Alpine districts of Germany, Switzerland, and Italy.
Source: Charles Holtzapffel, Descriptive Catalogue of the Woods commonly employed in this Country for the Mechanical and Ornamental Arts , London: Holtzapffel, 1843, as cited in:
Comment by the editor of the source quoted above, Hogg's Weekly Instructor:
This book is really valuable, but it is evidently almost entirely unknown even to those engaged in the investigation of the subject of which it treats. It is, moreover, the only one on the subject.
While Charles Holtzapffel was publishing woodworker's manuals on turning for amateurs as early as 1846, the source cited in the box below is the earliest manual on general woodworking that we have located:
The inclination or angle at which the iron is fixed in the stock is called the "pitch". The pitch is regulated according to the nature of the wood on which the plane is to be used. If the workman be using material of a hard and close nature, the iron must have much less inclination than would be correct for soft woods, such as deal or pine. The pitch cannot be varied in ordinary planes, therefore two or more planes, each having a different pitch, must be provided, if the work demands much change in the character of the wood.
The lowest, or common pitch, for surfacing-planes for soft woods, like pine or deal, is forty-five degrees from the horizontal line of the sole; York pitch, or fifty degrees, is suitable for mahogany, rosewood, &c; middle pitch, or fifty-five degrees, is the general angle for irons of moulding-planes for soft woods, and is also correct for smoothing-planes for hard materials; half-pitch, or sixty degrees, for moulding-planes for mahogany, rosewood, and other woods which are liable to " tear up." Still harder substances, such as boxwood, ivory, etc, require planes in which the pitch entirely disappears, the iron being fixed vertically. In some instances tho pitch is reversed by inclining the iron a little in the opposite direction. Both these planes are properly scrapers, and they only remove the material in the shape of dust.
Source Our workshop, being a practical guide to the amateur in the art of carpentry and joinery ... New York, T. O'Kane, 1873, page 25. (This book shows a New York publisher, but, internally, you soon detect references to things that are defintiely British, including a cross-reference to Charles Holtzapffel, cited above)