Copied from Nix's annotation on 23 June 1662:
1. 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.
In the timber trade, 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. In N. 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.
The word was introduced with the importation of sawn boards from some Low German district, and, as these consisted usually of fir or pine, the word was from the first associated with these kinds of wood.
1402 in C. Frost Early Hist. Hull (1827) App. 6 Mari Knyght de Dansk..xvj deles, iijm waynscots. Ibid. 18, iij dusen deles. a1450 Rature (in Hull Trin. House Records), Item for euerie hundreth of firre deales, xijd. 1558 Wills & Inv. N.C. (Surtees) I. 183 Ffyrdells of the biggest sorte..litle firdells..doble firr sparrs. 1583-4 Bk. Accts. Hull Charterhouse in N. & Q. 6th Ser. VIII. 217/1, 7 deals to seale the windows. 1595 A. DUNCAN Appendix Etymol., Asser, a deele or planke. 1604 Vestry Bks. (Surt.) 283 For fortie firre dales, xxiijs. iiijd. 1641 BEST Farm. Bks. (Surtees) 111 Robert Bonwicke of Wansworth demanded for everie deale a pennie, for bringing them from Hull to Parsonpooles, alledging that everie deale weighed three stone. 1762 STERNE Tr. Shandy VI. xxiii, A little model of a town..to be run up together of slit deals. 1820 SCORESBY Acc. Arctic Reg. I. 141 These huts, some constructed of logs, others of deals two inches in thickness. 1886 Law Times LXXX. 212/1 To there load a cargo of deals.
b. (Without a or pl.) Wood in the form of deals.
a1618 RALEIGH Obs. in Rem. (1661) 180 The huge piles of Wainscot, Clapboard, Firdeal, Masts, and Timber..in the Low-countries. 1627 CAPT. SMITH Seaman's Gram. ii. 14 Laying that Decke with spruce Deale of thirty foot long, the sap cut off. 1667 PRIMATT City & C. Builder 85, A handsom Door, lyned with Slit-deal. 1794 Builder's Price-Bk. 41 Whole deal dove-tailed dado. 1876 GWILT Encycl. Archit.