autofellatio
Vulgar slang; literally 'solo shakuhachi', a euphemism for the sexual act. Used in very informal, often jocular contexts.
I saw the term 'hitori shakuhachi' on an internet forum.
Compound of 一人 (hitori, 'one person') and 尺八 (shakuhachi, a Japanese flute), likely from the visual similarity between playing the flute and the act.