The story
From diaper + party, because most dadchelor parties restrict the giving of gifts to just diapers as opposed to baby showers in which different types of babycare items are gifted.
Etymology from Wiktionary, CC BY-SA 4.0.
diaper party
Today it means: Synonym of dadchelor party (“a party for a man whose partner is expecting a child, similar to a baby shower, especially one in which the gifts given are exclusively diapers”). Full definition →