Sounds lovely!
... and that’s an interesting theory, James. Wouldn’t it be wonderful if we’ve had such an establishment in Bedlington! Unfortunately, I think there are a few holes in the theory.
The Old English (OE) period in the development of the English language was 700 – 1100 AD. That’s more or less the period from the arrival to Britain of the Vikings through to the arrival of William the Conqueror and long before 1739. According to the Oxford Dictionary of English Etymology (ODEE) there was already during that time a word for wine in the English language – win (pronounced ‘ween’). This was the same word in Old Norse and in Old High German and it seems to have been part of the normal development of the word winam in the Germanic languages – from which English is descended.
At that time, English also had a word for Mead – meodu - and even this seems to have arisen as part of the natural progression of the Germanic word meduz through the same group of languages as seen in: Old High German - metu or mitu and Dutch – mede. (Point of interest: It’s from the same source that we get the word methylated).
This begs the question, why did English introduce sack as another word for wine?
According to the same source, ODEE, the first recorded use of the word sack, when related to alcoholic beverages, is found in writings from the sixteenth century. (In its relation to the coarse textile (sackcloth) or the bags made from it, it appears a couple of hundred years earlier). However, in relation to alcohol, the meaning of sack was very specific. It didn't relate to just any fortified wine but related generally to a specific "class of white wines from Spain and the Canaries” and the original spelling would have been “ (wyne) seck” meaning dry (wine). ODEE goes on to say that [seck] may originally have been “applied to wines of the sherry class, but later applied to others”. The Sherry connection would cover your theory on fortified wines; however, it seems that sack referred not to the wine itself but rather to one particular quality of wine – dryness. You might recognize it today as sec on French wines or seco on Spanish wines.
The alteration from OE seck to modern English sack is not explained but there are, throughout the history of the development of our language, many instances of changing vowel sounds in speech which lead to changes in spelling. Too many to go into here.