Sherry sack, at one time given by the Bristol people to their
“This metaphorical milk, whereby Xeres or Sherry-sack is
intended.” —Fuller. Worthies.
Mineral waters of Clifton, near Bristol, with a temperature not
exceeding 74; formerly celebrated in cases of pulmonary consumption.
They are very rarely used now.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894