a full moon. Reference to Remus Lupin in the Harry Potter series who is a werewolf. When the moon is full, he transforms, which is said to be painful, so when you see a full moon, you say "poor lupin". can also be used at a noun i.e. instead of saying "there's a full moon tonight," you'd say, "there's a poor lupin tonight". Sort of the opposite of a Lucky Lupin (crescent moon)
bob: Wow! look at the full moon, it's bright!
joe: yeah, but poor Lupin
bob: I really hope there's a poor Lupin tomorrow for the camping trip!