Autofellatio is a form of masturbation in which a person orally stimulates their own penis. Although there is plenty of lore - both modern and historical - surrounding this form of self pleasure, the actual practice is believed to be extremely rare because most people are physically unable to do it. The act is both an aspiration and an insult; while it's considered a bit of a holy grail of self pleasure, the need or desire to do it also implies that a person does not have a willing partner to do it for them.
Alfred Kinsey reported on the behavior of autofellatio in "Sexual Behavior in the Human Male," and while he noted that "a considerable portion" of males attempt this, he estimated that only two or three out of every thousand would actually be able to manage it.
Autofellatio is related to autocunnilingus, which is believed to be even more unlikely as an actual sexual practice.