A seventh son used to be so dubbed from the notion of his being intuitively skilled in the cure of agues, the king's evil, and other diseases.
“Plusieurs croyent qu'en France les septiennes garçons, nez de legitimes mariages (sans que la suitte des sept ait, esté interrompue par la naissance d'aucune fille) peuvent aussi guerir des fievres tierces, des fievres quartes, et mesme des ecrouelles,après avoir jeûne trois ou neuf jours avant que de toucher les malades.” —Jean Baptiste. Thiers: Traité des Superstitions, etc., i. p. 436.
(The). The cook on board ship, who “doctors” the food. Any adulterated or doctored beverage; hence the mixture of milk, water, nutmeg, and a little rum, is called Doctor; the two former ingredients being “doctored” by the two latter.
(The). 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.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894