One for the road
A final drink taken just before leaving on a journey.
'One for the road' sounds as though it might be a 'ye olde' expression used by medieval travellers when leaving an inn or by a Dickensian character taking a swig of grog before disappearing into a London fog. There is a suggestion that this phrase derives from the supposed practice of offering condemned felons a final drink at pubs on the way to the the place of public execution in London - The Tyburn Tree, but this isn't supported by historical record.
In fact, it is a 20th century phrase and came into being around the time of the outbreak of the Second World War. However, the 'for the road' element does have its genesis in earlier days.
Of course, travellers haven't always had the opportunity to stop for a meal or a drink whenever it suited them. In 18th century England food outlets would have been few and far between. If travellers wanted to eat on their journey they had to take their food with them. Whatever provision one made for one's journey was said to be 'for the road'. An early example of that in print is found in a journal called The Beauties of All the Magazines Selected for 1763:
The chief shepherd gives them three shillings in April, and three shillings in October, by way of regale for the road.
The first uses of 'one for the road', in which the 'one' is always an alcoholic drink, are found in print in the 1930s (and of course you can also have two, three or however many). An early example is found in the Edinburgh Evening News, August 1939, which reported on a debate in the British House of Parliament:
“ONE FOR THE ROAD”
Lady Astor asked if the Prime Minister would bear mind the fact that even a moderate quantity of alcohol was dangerous.
Captain Wallace [The Minister of Transport] replied: I have read the report and know very well that the noble lady is not favour of what is known as "one for the road".
The quotation marks around the phrase indicate that it wasn't fully integrated into the language.
When Lady Astor made her plea for moderation in drinking, those taking 'one for the road' were most likely to be travelling on foot. These days most people leaving pubs travel by car. Nevertheless, and despite numerous campaigns for a zero alcohol limit for drivers, the UK drink-drive limit is still set at a level that allows 'one for the road'.
Other 'One' phrases: