Hi and thank you for your question. Nicknames, like "11s" when referring to the furrow or vertical creases that form between the eyebrows, can often become used in an industry and it takes a good question like this to stop and realize how this so commonly used reference internally, can cause you to speak to patients using this terminology (and in this case, Allergan, the makers of Botox) without considering how confusing this can be.
As others have answered, the "11s" has become a name given to very common vertical surface lines between the eyebrows, as the strong muscle activity underlying contracts and causes repetitive "creasing" or "pinching" of the skin in that area. There is a patient education tool provided by Allergan in many offices that is attempting to answer this very question, offering a question to you as to if you have a "1", "11", or "111", as there are similarities in many people as to how the muscles work in this area. Some people will notice a single vertical line or pinch, and sometimes these muscles crease the skin to form as many as three creases, all which can be softened, relaxed, and restored to a smoother appearance by administering Botox to the underlying muscle.
On behalf of all of us in an industry where this slang has become everyday use, we hope this is helpful clarification.