Certified Nurse's Bitch. It's a Certified Nurse's aide who works in a hospital and the nurses are having him/her do everything, including simple things, when the unit is super busy.
Generally, it's the lazy nurses who don't want to do the easy work (i.e. retrieve water for a patient)
Nurse Betty is told by a patient that she wants some water. Instead of Nurse Betty getting it, she tracks down Danny and asks him to get it. Danny tells Sheila "I'm not a Certified Nurse's Aide anymore, I'm a C.N.B!" Sheila asks "What's C.N.B.?" Danny tells her "Certified Nurses bitch!"