LOST WEEKEND. BtVS Intertext/Allusion.
In "The Dark Age" Buffy and Giles engage in the following exchange:
BUFFY: Giles, what's going on?
GILES: It's complicated, Buffy, and quite frankly, it's private.
BUFFY: I don't care from private! I care from dead guys attacking us. I care from you lost weekending in your apartment.
GILES: I wasn't... I was just trying to find a solution.
BUFFY: Giles, share! What is the Mark of Eyghon?
GILES: Hey! This is not your battle! And as your Watcher, I'm telling you unequivocally to stay out of it! I have to get Jenny home.
The Lost Weekend was a 1945 Billy Wilder film about a man (played by Ray Milland) battling alcoholism.