One for the road
Definition 1 of 1
Pro Tip 1/3Social Custom
Used in social gatherings where having a last drink is customary before departing. 
Everyone was leaving the party when Mike shouted, 'How about one for the road?'
Pro Tip 2/3Informal Setting
The phrase is casual and mostly used among friends or in informal settings. 
After a long dinner with college buddies, Sam suggested having one for the road.
Pro Tip 3/3Not Literally
It's not always about drinking; it can be used metaphorically for any last item before leaving. 
Though we were full, we still grabbed a cookie for the road.