means The meal eaten in the middle of the day, between breakfast and dinner.

from A shortening of 'luncheon,' which itself likely grew from 'lunch' meaning a thick piece or hunk of food — possibly related to 'lump.' There's an older word 'nuncheon' (a noon drink or snack) hovering in the background too, and the two may have nudged each other along. Either way, what began as a casual bite between meals climbed its way up to a meal in its own right.