In English language, literally, sheaves refers to an amount of stalks and ears of cereal plants that are bound together as a bundle. Cereal plants are usually packed in this manner after harvest. Sheaves generally refers to a bundle of things that are tied together.
The word or phrase that best defines the italicized word in <em>"And by the moon the reaper weary, Piling sheaves in uplands airy" </em>would be bundles of grain.
he figured out the meaning by seeing it looked like the word admire? (im sorry, im not sure on this one but i am trying to help out, im truly sorry if i am not right)