One for the road
A final drink taken just before leaving on a journey.
The 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, isn't supported by historical record. The phrase isn't known until the mid 20th century, long after Tyburn ceased to be a place of execution. It appears to have originated just as a colloquial reference to a departing drink in English pubs, just as 'a quick one' refers to a one taken in haste.
The earliest citation I can find of 'one more for the road' is from The Times, March 1939:
"Propaganda should be employed to train and fortify public opinion in the condemnation of persons who drink before driving - above all to discourage the practice of 'one for the road'."
That piece is as reproachful as a second citation from the same year is comical. This report, of a court case in England, was reported in the Canadian newspaper The Lethbridge Herald, in December 1939:
A cultivated English voice which said, "Come on, let's have one for the road." led a Weymouth innkeeper into court. Police outside his inn heard this remark - after closing time. So Landlord Frank Roe received a summons, The landlord's father explained in court that police had heard the English-speaking announcer of the German Zeesen radio station popularly known as "Lord Haw Haw." The summons was dismissed.
The phrase really took hold when Johnny Mercer used it in the lyrics of his song One for My Baby (and One More for the Road), which he wrote for Fred Astaire in 1943:
Its quarter to three
There's no one in the place 'cept you and me
So set em up Joe
I got a little story I think you oughta know
Were drinking my friend
To the end of a brief episode
So make it one for my baby
And one more for the road
The phrase has proven popular as a title. It has been used for a 1984 Harold Pinter play, a Stephen King short story and an album title by The Kinks (1980), April Wine (1984), Trouble (1994) and Ocean Colour Scene (2004). It was also the name of a 1995 British television series, starring Alan Davies.
The Australians, ever linguistically inventive, prefer to have 'one more for the bitumen'.
See also one over the eight.