1
general::
Phrase(s): *one for the road
a drink; a drink before a journey or before leaving a bar. (*Typically: down ~; drink ~; have ~; take ~.) • Let’s have one for the road. • Don’t down one for the road if you are going to be the driver.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
A final drink before leaving, as in Won't you have just one for the road? This term always alludes to an alcoholic drink and a practice that, if the person is going to drive away, is not only frowned on but in many places illegal. [First half of 1900s]
American Heritage Idioms