Not sure if this is a joke or not. I would have to see it used in its context. But I understand it. It is absurd that the lock-keeper would want some plaster from the blood donor. Since the blood donor needs the plaster more than the lock-keeper. So I think it is used more in a metaphorical sense. Maybe to mean.....no matter how much a part of him wants help from the other, it just cannot or it won't help. That's what I could get from it

cozy, It's from the movie Hotel du Nord. to plaster.....to apply a bandage or ointment; (figuratively) to apply a medicine or remedy to be plastered.......(slang) to be very drunk The man has two jobs. He is a lock-keeper and a blood donor. The lock-keeper would like some wine as a medication. The blood donor can't drink alcohol because it would poison his blood.