How do individuals in twelve-step fellowships like Alcoholics Anonymous (AA) and Narcotics Anonymous (NA) interpret and enact "anonymity?" In this paper, we answer this question through a mixed-methods investigation. Through secondary analysis of interview data from 26 participants and an online questionnaire (N=285) we found three major interpretations of anonymity among AA and NA members: "unidentifiability," "social contract," and "program over individual." While unidentifiability has been the focus of computing investigations, the other interpretations provide a significant and novel lens on anonymity. To understand how and when the unidentifiability interpretation was most likely to be enacted, we conducted a quantitative analysis of traces of activity in a large online recovery community. We observed that members were less likely to enact "unidentifiability" if they were more connected to the particular community and had more time in recovery. We provide implications for future research on context-specific anonymity and implications for design in online recovery spaces and similar sensitive contexts.