Not Know Which End Is Up Meaning

(idiomatic) To have no common sense; to be ignorant of the most basic facts; to be very confused.

Example: 2009, Lis Wiehl and April Henry, Face of Betrayal, ISBN 9781595547057, p. 145 (Google preview):
  â€œI'm not some naïve, virginal little nothing who doesn't know which end is up.”