Marrowfat peas are green mature peas that have been allowed to dry out naturally in the field, rather than be harvested whilst still young like the normal garden pea. They are used to make mushy peas and also the snack food wasabi peas.
Marrowfat is a traditional, starchy, large-seeded variety of pea (Pisum sativum var. medullare.) The word was coined around 1730 from marrow + fat.