E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Brown sherry, so called because it is concocted from a harsh, thin wine, by the addition of old boiled mosto stock. Mosto is made by heating unfermented juice in earthen vessels, till it becomes as thick and sweet as treacle. This syrup being added to fresh must ferments, and the luscious produce is used for doctoring very inferior qualities of wine. (Shaw: On Wine.)
To doctor the wine. To drug it, or strengthen it with brandy. The fermentation of cheap wines is increased by fermentable sugar. As such wines fail in aroma, connoisseurs smell at their wine. To doctor wine is to make weak wine stronger, and sick wine more palatable.