The usual dictionary definition for this now rather rare word — “treat roughly or severely” — hardly does justice to the history of a slang term that has had several meanings. Its origins lie in the eighteenth century in Britain, though where its first users got it from remains a mystery. The experts hazard a guess that it was probably a fanciful conflation — suggestions include stifle + suffocate and spill + castigate. You can spell it with one f or two, as the fancy takes you, though when it first appeared it had only one.
Over half a century, it rapidly developed from its initial sense of “confound, silence or dumbfound”, through “handle roughly or treat severely”, to “crush, destroy or kill”. T W E Holdsworth borrowed the last of these in Campaign of the Indus of 1840: “Of the enemy, about 500 were killed, and more than 1500 made prisoners; and of the remainder, who made their escape over the walls, the greater part were cut down by the Dragoons, or spifflicated by the Lancers.” Despite these gory associations, by about 1900 it had softened in Britain into a jokey term for some unspecified but vaguely unpleasant punishment with which one might threaten a naughty child (“I’ll spifflicate you if you won’t be quiet!”).
In America at around the same date, the word took on another sense still, that of being drunk. An early example is from the sporting section of the Washington Post of July 1904: “They forced his teeth open, and, while a couple of them sat on his chest, they poured about a quart of corn liquor into his system. He was so spifflicated before they let him up that they had to lift him bodily and plant him in a seat.”