Completely, totally, all of it. The expression is often phrased as to swallow [something] hook, line, and sinker, alluding to the gullibility of a fish that takes in bait so completely that it swallows the fishing hook, line, and sinker as well. The term originated in the United States in the mid-nineteenth century. In the late 1980s Len Deighton used it in the titles of a series of three espionage novels involving complicated deceit, Spy Hook, Spy Line, and Spy Sinker. See also lock, stock, and barrel.