The origin of the word lunch seems to be from a word meaning a "chunk, thick piece of food", possible lump. B&M OED suggests it was based on the perceived relation between such pairs as hump/hunch and bump/bunch. The relation with luncheon may be that luncheon was based on an anaolgy with punch/puncheon and trunch/truncheon. The definitions given for lunch are a mid-day meal or a light meal between two more substantial meals.