The etymology isn't difficult. It's just deserts/what he deserves. Deserts/deserves, geddit? Desserts can only ever mean puddings/sweets. Just desserts means that you go to a restaurant and get told that all they have is ice cream, chocolate brownies or spotted dick with custard. Speaking of a baddie who you hope will get his come-uppance as getting his just desserts is just wrong, and always was!