É uma referência a como Hemingway cometeu suicídio atirando em si mesmo com uma espingarda. Como wikipedia diz sobre o assunto:
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".
A piada atual é mais um tema adulto, então vou deixar uma explicação fora da resposta. Para ser um pouco mais explícito (em spoilers, porque uma vez que é um pouco mais claro, é mais uma área NSFW)
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.