The story
One theory regarding the origin of the name whoopie pie is that it arose from a production of the musical play Whoopee! in Boston in the late 1920s in which chocolate pies were thrown out to the audience during the song "Makin' Whoopee". Alternatively, the pie (which is believed to have originated in Pennsylvania Dutch Country) is said to have gotten its name when Amish women made them for their husbands and children, and they were so delighted that they exclaimed, "Whoopie!"
Etymology from Wiktionary, CC BY-SA 4.0.