Posted by Robert Dilley on September 23, 2002 at 14:16:49 from 65.39.15.64 user rdilley.
Complete OED to the rescue again. Jill "of a boat: to move about; to move around; to idle about."
Presumed to be a variant of gill. Readers are specifically referred to a mid-19th century quote "gilling -- drinking half-pints in different places". Obviously not a very respectable derivation!
The quotes supporting the nautical "jill" are from (the earliest, 1955) The Times; William Golding (Pincher Martin); Francis Chichester and (the most recent, 1972) The Guardian. Nothing from AR, who was using it before any of them.
On the lines of swigging, I should mention Terry Pratchett's definition of quaffing -- "It's like drinking, only you spill more."