A meal taking the place of both breakfast and lunch. Unlike "brunch" which is usually a hearty, planned affair in a restaurant, lunkfest usually consists of fast food or whatever food is lying around the house, and is usually the result of sleeping in to the point where eating a separate breakfast and lunch would be impractical.
Guy: I overslept.
Other guy: That sucks, you missed breakfast.
Guy: Who cares? I can still eat lunkfest.