This article is from the Kurt Vonnegut FAQ, by George A Cooley firstname.lastname@example.org with numerous contributions by others.
A favorite quote among at least a faction of the newsgroup readership,
the quote is thought to have originated in "Slapstick" (1976.) It
appears numerous times in that book, and actually becomes a bit of a
plot point. The quote is "why don't you take a flying fuck at a rolling
doughnut? Why don't you take a flying fuck at the mooooooooooooon?"
(pp 163, among others.) But it was also found (by Kevin Brophy
(email@example.com) in "Slaughterhouse-Five." "'Go take a flying fuck at a
rolling doughnut,' murmered Paul Lazzaro in his azure nest. 'Go take a
flying fuck at the moon.'" (pp 147) This is the earliest known
appearance of the quote to date.