An acronym: Bare Back Ass To Mouth.
Slang mainly in prostitution circles and used to describe what a particular call girl will do. In this case Anal sex without the use of a condom followed immediatly by oral sex.
An actual craigslist ad:
Let this young lady charm your heart with all her special talents. A brunette with blue eyes, who could resist such a beauty. Availiable appointments now.
$300/hr- full service