A slang expression referring to human flesh used as meat. Sometimes you'll see "long pig" instead of "long pork" used the same way.

Apparently the term originated in the South Pacific, and comes from the alleged resemblance of this meat to pork. For example, you'll find in chapter 6 of Robert Louis Stevenson's In the South Seas:

"We had been but three days in Anaho when we received the visit of the chief of Hatiheu, a man of weight and fame, late leader of a war upon the French, late prisoner in Tahiti, and the last eater of long-pig in Nuka-hiva."

Another illustrative quote comes from Chapter 17 of William B. Churchward's My Consulate In Samoa:

"Whilst strolling about the town to-day (the 9th), I was called in by the British doctor to see a curious specimen of a Samoan patient he had brought in to him for treatment, and, on entering, saw a fine-looking old man sitting on the floor with his eyes not only close shut, but forcibly kept so, with all the effort the patient could muster. He was surrounded by four strong friends, who appeared to be acting as a sort of police over him, and ready to seize upon him whenever required. It was a most strange case, and had puzzled our Aesculapius not a little. This respectable old gentleman - all right in every other respect - had suddenly taken into his head that he should like to resume ancient Samoan customs, and hungered for human flesh to vary the monotony of his yam and taro. It appeared when at home he would sit in his house like a spider in his web, and every fat boy whom he saw pass he would hunt all down the town with wild howls and watering mouth, seeking to convert the well-favoured youth into the 'long pork' of his ancestors for the satisfaction of his horrible craving. He at last became a regular nuisance; no baby was safe anywhere while he was about, and the mothers were in continual apprehension of coming home one fine day and finding the creature polishing off the last wing-bone of their latest treasure. The point of forbearance was exceeded when he, not being able to catch the tender youth-spoiling all round the village for want of cooking-suddenly attempted to devour an old and trusted friend raw. Reviving cannibalism they could put up with to a great extent, but to improve upon the old system and take the rations uncooked could not be endured; so he was captured, and taken in to the 'Fomai Peritania' (British doctor) to see what he could do in this strange case. The man, as I said before, was quite sensible on all other subjects. and the reason that he kept his eyes shut was-he told me himself-to prevent his seeing the. doctor, for he knew that if he looked upon him he should want to dine off him. What became of this interesting patient on his return I never heard."

Churchward discusses whether cannibalism was actually commonly practiced in Samoa, and concludes that there was ample historical evidence, although it had fallen out of fashion by the time he was writing in 1882.