CSF
From Gomerpedia
CSF (or cerebrospinal fluid) is that magical fluid you can never get on a first attempt of a lumbar puncture.
Dialogue Between Provider & Patient During LP
Provider: Then once you're positioned, I'll numb the area and we'll give it a go!
Patient: Sounds good. (90 minutes later.) OWWWW!! Aren't you done yet? I can't feel my legs!
Provider: Just another 15 attempts and I'll get it. (Mutters under breath.) I f**king hate LPs.