1Designating a system or arrangement (as in a self-service restaurant or shop), in which one helps oneself to food or drink, goods, etc.
2Of a restaurant, shop, etc.: in which customers serve themselves instead of being attended to by a member of staff.
3Of a meal, food, or drink: to which one helps oneself, as from a buffet at a party, or at a self-service restaurant.
Early 20th century; earliest use found in Indianapolis News. From serve + yourself.