"It may well be the only one in existence," said auctioneer John Ferguson, of Cluny Auctions in Buckie, who admitted it is impossible to put any kind of price on it.
He added: "So little is known about the whisky - it will be worth simply what someone is prepared to pay for it on the day of the auction.
"No reserve price has been put on the bottle, so it will sell.
"I would like to see it go to a good home - a museum or a serious collector. I think it would be past drinking."
The original label on the unusual 10-inch, corked bottle says it is finely matured Glen Boyne old Scotch whisky. At the bottom of the label it has the retailer's name G. G. McRobie, Portsoy, established 1846.
Mr Ferguson told the 'Banffshire Journal' this week: "The bottle has been passed down through the generations of a local family.
"I have tried to find out more about the whisky, but with little luck.
"I know there was a retailer by the name of McRobie in Portsoy, but I believe the shop has been closed a long time.
"It is my guess the whisky dates to around the turn of the 19th century, and certainly pre-Second World War.
"I would say it is the most interesting item for my whisky auction which will start at 11am on Saturday, October 31. I expect around 200 people turn up.
"People will also be able to follow the auction online."
In an effort to find out more about the whisky, the 'Banffshire Journal' sought the help of two Portsoy men with a keen interest in the town's history, Findlay Pirie and John Mitchell.
Mr Pirie (85), a retired Banff postmaster living in Portsoy, said William McRobie, originally from Keith, opened a business in Portsoy as a grocer and spirit merchant.
"He added branch after branch until he became the largest employer of labour in the district," said Mr Pirie. "He was also a fish curer, coal merchant, shipowner and had the Fordyce limeworks and Durn quarries.
"He died in 1886, and George, the son, was the name on the family shop when I was young. It was on the corner of Bridge Street and Low Street.
"He sold two types of whisky: one was more expensive than the other, but there was no longer a distillery in the town by then. The name Glen Boyne would have been taken from the nearby Boyne Burn to give it a local name. There is not a glen as such in the area.
"I think the shop closed around the 1950s or 60s. I am not aware of any McRobies still living in the area."
Mr Mitchell said: "I cannot be certain but it is my belief that there was no distillery of the McRobie name. G. G. McRobie had extensive interests in ship chandlery, was also a coal merchant, as well as being a grocer and spirit merchant.
"It would seem probable that he purchased whisky from some local source and bottled it under the name Glen Boyne There had been a distillery in Portsoy, but there is nothing to suggest that it might have been the supplier.
"The 'Burnside Distillery', according to the late local historian Jim Slater, was situated close to the junction of Burnside Street and Culbert Rig. There being no railway line at that time, there was no Bridge Street.
"The person running the distillery was called William Morrison, and it was also a William Morrison who, in 1874, acquired the 19-year lease for Craigmills Farm, the site for the Glenglassaugh Distillery, from R. W. Duff of Glassaugh.
"This could have been the William Morrison, brother of Alexander, one of the founding partnership in that distillery."