A variation of "poontang."
Sex with someone you are not morally supposed to have sex with. This includes minors, married persons, members of the clergy, incest, and sex between stepsiblings.
The term is derived from the ABC television show LOST in which stepsiblings Boone (Ian Somerhalder) and Shannon (Maggie Grace) have sex.
Irony: The term "Boone" is actually French for "good," making "Boonetang" actually mean "good poon." In usage, however, it is "naughty poon."
"Shannon got herself some hot Boonetang in Sydney."
"I'm stopping by the church for some Boonetang."
"That 15-year-old is one nice piece of Boonetang."