A phenomena of the pagan revival in 18th century, which combined two popular trends sexual libertinage and the kind of paganism seen in the the neo-Druid orders of the period. While this had a profound effect on some, including William Blake, it was more often just an excuse for debauchery. It may have indeed caused Priapism in the sense above :-) The Hell Fire Club was a paradigm case of a priapic cult.