Yes, it's the sherry type of fortified wine from Spain. It was a common generic term in the past, but is now dated. Some brands may still use it in their brand names however.
I think it's quite strong, so a tankard of the stuff sounds a bit much! Is the source from the 17th C. or is merely set there. I'm just wondering if sack would be drunk from a tankard, normally used for much weaker beer.