Det är en hänvisning till hur Hemingway begick självmord genom att skjuta sig med ett hagelgevär. Som wikipedia sägs om ämnet:
Two days later, in the early morning hours of July 2, 1961, Hemingway "quite deliberately" shot himself with his favorite shotgun. He had unlocked the basement storeroom where his guns were kept, gone upstairs to the front entrance foyer of their Ketchum home, and according to Mellow, shot himself with the "double-barreled shotgun that he had used so often it might have been a friend".
Det faktiska skämt är mer av ett vuxentema så jag kommer lämna en förklaring utifrån svaret men. Att vara lite mer explicit men (i spoilers för att en gång det är lite mer stavat ut är det mer av ett NSFW-område)
it's important to note that it is Beth who wants to be Hemingway and Jerry's penis is the shotgun. Hemingway shot himself in the head to commit suicide and Beth has just shot some cronenbergs in the head. If you still can't work the joke out from that you're too young and it's not exactly appropriate for this site to spell it out.