If sacke and sugar be a fault, God helpe the wicked.
A sort of fortified white wine, likely relatively sweet, imported from Spain and the Canary Islands, very popular in England during the 16th and 17th centuries. Matured in wooden barrels for times not upwards of two years. Sherry is its modern equivalent, its name originating with Sherris sack imported from Jerez de la Frontera. Shakespeare's Falstaff was a big fan - the wine is namedropped twenty-one times in connection to him in Henry IV, Part 1.
Samuel Pepys's diary makes mention of it, too, in its October 15, 1665 entry:
By and by by appointment comes Mr. Povy’s coach, and, more than I expected, him himself, to fetch me to Brainford: so he and I immediately set out, having drunk a draft of mulled sacke;
There's some confusion about where the term sack comes from. The OED places its earliest English use in the early 16th century as wyne seck, and elaborates on possible associations with the French vin sec ('dry wine') and the German sekt, though with some uncertainty. The Oxford Companion to Wine pitches a different theory by proposing origins in the use of sarcas to mean exports of wine, from the Spanish sacar ('to draw out').
References:
OED
http://www.winepros.com.au/jsp/cda/reference/oxford_entry.jsp?entry_id=2790
http://www.pepysdiary.com/archive/1665/10/15/
http://en.wikipedia.org/wiki/Sack_(wine)
Henry IV