Not surprisingly, the term “Boston Massacre” has been used frequently to describe both individual games and whole series involving Boston teams over the years. The earliest examples of both that I’ve found actually involved the Boston Braves, not the Red Sox. [Ken Liss]
“Having tucked 10 straight safely away the Giants resumed their even tenor in the second Boston massacre, with George Wiltse facing Otto Hesse in a battle of southpaws.” San Francisco Call, June 29, 1912, p17
— From a report on a double header in New York. The Giants beat the Braves 10-3 in the first game and 12-3 in the second. These were the 10th and 11th consecutive wins for the Giants in what became a 16-game winning streak. The streak had started against the Braves in Boston with a five-game sweep that included scores of 21-12, 17-5, and 14-12. It’s not entirely clear that the term referred just to the two games rather than to the two series.
“Their work indicated that the unpleasant nightmare is over. The Boston massacre is a thing of the past. The Philadelphia jinx is trampled under Corsair hoof.” Pittsburgh Gazette Times, May 15, 1913, p10
— Reporting on the Pirates beating the Phillies to end a seven-game losing streak that started with four losses to the Braves in Boston followed by losses in the first three of a four-game set in Philadelphia. None of the losses in Boston were routs. Three were one-run games, and two of those were decided in extra innings.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
Though not specifically stated as the first use, the earliest example given for the fourth sense — “Any egregious defeat or series of defeats incurred by the Boston Red Sox at Fenway Park” — is 1951. The first through third senses all refer to a specific season (1978) or a specific series sweep (September 1978 and August 2006).