There’s a word tare, meaning “The weight of the wrapping, receptacle, or conveyance containing goods, which is deducted from the gross in order to ascertain the net weight” (OED), that I’ve looked up any number of times but never remember because it’s not part of my mental world. (If you’re curious, it’s via French from Arabic ṭarḥah ‘that which is thrown away,’ from ṭaraḥa ‘to throw (away),’ which is also the root of mattress, from Arabic maṭraḥ ‘place where something is thrown, hence carpet, cushion, bed.’) Today, reading a fascinating 1986 interview (in Russian; found at Avva) with the manager of a fruits-and-vegetables store that throws a great deal of light on the realities of doing business in the late-Soviet period, I hit the word тара [tara], looked it up, found it meant tare, cursed, looked that up, and got the definition above, which I think may finally stick. But looking through the OED entry I found the following phrase:
tare and tret: the two ordinary deductions in calculating the net weight of goods to be sold by retail: see TRET; also, the rule in arithmetic by which these are calculated.
So I saw TRET, and here’s what I found:
An allowance of 4 lb. in 104 lb. (= 1/26) on goods sold by weight after the deduction for tare.
The reason or ground of the allowance was apparently forgotten already in the 17th c., and has been variously given since: see quots.
(“Origin and history obscure.”) Some of the various explanations:
1670 BLOUNT Law Dict. s.v. Tare and Tret, The other [Tret] is a consideration allowed in the weight for wast, in emptying and reselling the Goods.
1678 PHILLIPS (ed. 4), Tret, a certain allowance that is made by Merchants, before a Commodity is garbled from its refuse [1706 ed. Kersey adds] as Dust, Moats, &c., which is always 4 in every 104 Pounds.
1882 BITHELL Counting-ho. Dict., Tret, an allowance made for wear, damage, or deterioration in goods during transit from one place to another.
Another citation mentions cloff, which is “An allowance (now of 2 lbs. in 3 cwt., or 1/168), given with certain commodities, in order that the weight may hold good when they are sold by retail,” but de minimis non curat Languagehat.