Summer = beach = cooler. Stocked with bags of ice or frozen cold packs and—depending on the age cohort—sandwiches, lemonade, and watermelon chunks, or beer, beer, beer, and more beer, the squat plastic chests have been American picnic stalwarts since the 1960s. They originated as part of the Natick Center’s effort, beginning in 1955, to develop cellular polymers, foamed plastics, as building materials. The rigid, strong, and lightweight stuff was quickly incorporated into other uses, including refrigerated containers and insulated food coolers. Early contractors on Natick’s foamed plastics project included Dow Chemical and General Foods; their effort included at least two industry conferences, one in 1963 and the other in 1966.
Thanks to Richard A. Gabriel for this suggestion.