I came hoping to find the definition of "fritoon," as well, having also encountered it in O'Brian's work. I tried adding a T, subtracting an O, spelling it fritune, and even Frenchifying the spelling, to no avail. Too bad. Do you suppose the author made the word up at one time, and then began using it in his work? I have done that myself with my vocabulary, making up words and using them as though they had been vetted by the OED.