NIGHT CAP

 Last drink of the night before going home (or going to bed, if you are well behaved)

e.g.

But a nightcap is different from an after-dinner drink or digestif. I stick with the classics: top-shelf whiskey, good brandy (usually Cognac), ..

www.theatlantic.com/health/...nightcap/267437/

or

Let's have a nightcap before the cops arrive.