(From The Wrecker by Robert Louis Stevenson and Lloyd Osbourne, Chapter XII, published 1892)
You don't know; you haven't a guess. How would you like to go on deck for your middle watch, fourteen months on end, with all your duty to do and every one's life depending on you, and expect to get a knife ripped into you as you come out of your stateroom, or be sand-bagged as you pass the boat, or get tripped into the hold, if the hatches are off in fine weather? That kind of shakes the starch out of the brotherly love and New Jerusalem business. . . . No; it has an ugly look to it, but the only way to run a ship is to make yourself a terror.”
“Come, Captain,” said I, “there are degrees in everything. You know American ships have a bad name; you know perfectly well if it wasn't for the high wage and the good food, there's not a man would ship in one if he could help; and even as it is, some prefer a British ship, beastly food and all.”
“Oh, the lime-juicers?” said he. “There's plenty booting in lime-juicers, I guess; though I don't deny but what some of them are soft.” And with that he smiled like a man recalling something. “Look here, that brings a yarn in my head,” he resumed; “and for the sake of the joke, I'll give myself away. It was in 1874, I shipped mate in the British ship Maria, from 'Frisco for Melbourne. She was the queerest craft in some ways that ever I was aboard of. . .
What does but what mean in the sentence I don't deny but what some of them are soft? I suspect it is slang or informal and has to be I don't deny but that some of them are soft.