Sack is the forefunner – er, forerunner – of our modern sherry; a white fortified wine the medieval people imported from Spain or the Canary Islands. Most sack was probably a sweet wine as it was matured in wooden barrels for a limited time only. Today, we still see sack in some sherry names, such as “Dry Sack” (Williams & Humbert brand). We also have a tastefully named “In the Sack” cocktail you can make using cream sherry.
Closest to that medieval taste: A cheap medium Oloroso sherry. Example of a medium-bodied Oloroso sherry: Dios Baco Elite Oloroso Medium Sherry.