I think the author of the recipe might means one nutmeg seed that has been grated! They come dried, are not more than about an inch long, and hollow in the center. You can grate them finely so they basically turn into very aromatic and fresh powder. They sell them in the bulk section of spices at Whole Foods. One of them weighs 0.2-0.4 ounces, according to Wikipedia, so a lot less than one ounce of nutmeg.