a discrete item of work that will be hard to achieve, such as a business challenge whose solution will be burdensome to find or a question that will be difficult to answer; used to draw attention to effort that, while not precisely defined, is expected to be significant; calls to mind a macadamia or walnut whose edible portion can only be accessed through the use of a specialized cracking implement; similar to tough putt and tough row to hoe, all three of which focus on difficulty but with a lighter tone so as not to be seen as grumbling