The weak beer supplied to ships on the home station. A swipe is an implement for drawing water for a brewery, the name of which has thus been transferred to the beer.
Purser's swipes; small beer: so termed on board the king's ships, where it is furnished by the purse...
Dictionary of The Vulgar Tongue by Francis Grose
bad small-beer. The same as taplash. ...
A glossary of provincial and local words used in England by Francis Grose