Also, BS is a common abbreviation for "Bullshit", even though this connection is never made explicit in the show.
Maeby was using the fake illness to gather donations from her high school classmates and teachers. ("Altar Egos") She was chastised about the lie by George Michael, and planned on "killing off" Surely as to not raise suspicions. ("Justice Is Blind")
Surely later entered the Miss Inner Beauty pageant at the Church and State Fair, and although she never mentioned her BS, assumed she would lose due to her strange false nose and wheelchair. Instead, she won. ("Notapusy")