A glassful is as much liquid as one drinking glass will hold — one glass of something. “A glassful of water” names the amount, not the glass itself. It is a homely measure word, like spoonful or handful, and you will meet it with water, milk, wine, or medicine. Though actually, people do not always mean a glass filled right to the brim; it often just means “about one glass.” In everyday talk many people simply say “a glass of,” so glassful can sound a little careful or old-fashioned, but it is still ordinary in recipes and with children. You can have two glassfuls. The -ful here means “the amount that fills,” not “full of glass.”