Apart from being the title of a quite decent A Perfect Circle album as well as the number of the last step on the hangman's gallows, the "thirteenth step" is an euphemism for sexual predation within 12-step program groups such as Alcoholics Anonymous, Narcotics Anonymous and similar. Originally, the highly religious AA'ers used the term "thirteenth step" to refer to the death of a recovered alcoholic, taking the thirteenth and final step into the grave (from where he supposedly would meet his God).
The contemporary use of the term arose when AA came of age and some of the more sinister aspects of the groups began to show. To understand how sexual predation is a problem in a group like AA, it is useful to understand how newcomers are inducted into the
cult self-help group. When a newly sober alcoholic shows up at his/her first AA meeting, one of the first things to usually happen is for the newcomer and the group to select a sponsor who will help the neophyte with the steps of the program. Unfortunately, some of the program's steps are decidedly meant to destroy independence: Step 1 is admitting that you are powerless to control your own life, step 2 is to admit that you are insane, step 3 is to surrender your will and your life to God or the AA group. Sounds scary and sect-like? That's because it is. When a weak newcomer who's hit rock bottom (to use another AA term) takes the "fearless moral inventory" (confessing former sins) the 12-step program requires, it is usually the sponsor who's the listening part. Combine the independence-destroying exercises and this confession of a person's innermost secrets with an unscrupulous sponsor, and the opportunities for exploitation and manipulation would make a Microsoft executive rub his slimy paws in delight. If you take a random sample of drunks, I'll bet you a beer that you'll find a fair share of thieves, con artists and people of similar moral fiber in there, so it shouldn't really come as a surprise that some people read the steps and see opportunity. The people who serve as sponsors run the full gamut from sagely feel-good kind samaritans to dogmatic religious fanatics, neurotic control freaks, self-serving opportunist bastards -- and, of course, that subspecies of self-serving opportunist bastards, the sexual predators.
It goes without saying what some of these unsavory types would do with the power they gain over their sponsees. Recovered alcoholics seldom have much money to extort, and those brainwashed drones who have had a long career in AA often don't exactly function well in other social situations. Hence, abusing their position as AA sponsors is often the best chance they have of getting laid. Most AA groups have their share of sexual predators of both sexes (and sometimes both gay, bi and straight), meaning that while the stereotypical situation is an older male member taking advantage of a female newcomer, the opposite occurs at an alarming frequency too. Everyone's a target to a pack of these desperate freaks; man, woman, gay, bi or straight. While the AA organizations frown on the practice, it is still very widespread among their members.
The fact that it is so common that the AA'ers have invented a term for it should be raising alarms everywhere.