E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
After Meat, Mustard.
In Latin, Post bellum, auxilium. We have also, After death, the doctor, which is the German, Wann der kranke ist todt, so kommt der arztnei (when the patients dead, comes the physic). To the same effect is When the steed is stolen, lock the stable door. Meaning, doing a thing, or offering service when it is too late, or when there is no longer need thereof.