Someone who volunteers to keep an eye on someone who is intoxicated. They are always sober, or at least have control over themselves and are able to "sit" (babysit) the intoxicated person to prevent them from accidentally harming themselves (from falling and such). They should be someone trustworthy. This practice is recommended for strong mind-altering psychedelic drugs, such as Salvia, Phencycladine, Jimson Weed, and Ketamine.
Synonym: SS, SS Guard
Hey will you be my sober sitter? I really wanna try salvia.
Sure man, whatever.