An apothecary. Apothecaries are so called because they administer boluses. Similarly Mrs. Suds is a washer-woman; Boots is the shoeblack of an inn, etc.
George Colman adopts the name for his apothecary, who wrote his labels in rhyme, one of which was-
When taken, To be well shaken;
but the patient being shaken, instead of the mixture, died.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Bolus from Infoplease: