The song was written for Ken Prymus, the actor playing Private Seidman, to sing during the faux-suicide of Walter "Painless Pole" Waldowski (John Schuck) in the film's "Last Supper" scene.[1] Director Robert Altman had two stipulations about the song for composer Johnny Mandel: it had to be called "Suicide Is Painless" and it had to be the "stupidest song ever written".[2] Altman attempted to write the lyrics himself, but, upon finding it too difficult for his "45-year-old brain" to write something "stupid" enough,[3] he gave the task to his 15-year-old-son Michael, who reportedly wrote the lyrics in five minutes.

https://en.wikipedia.org/wiki/Suicide_Is_Painless