Know Which End Is Up Meaning

(idiomatic) To possess sound judgment or common sense; to have a clear understanding of a situation.

Example: 1992, Dorothy Garlock, Glorious Dawn, ISBN 9780446361828, (Google preview):
  He's so in love, he doesn't know which end is up.
2005, John C. Hefley, Failure Equals Death, ISBN 9781420829129, p. 29 (Google preview):
  â€œI'll gladly take criticism from a cop who knows which end is up and isn't afraid to do the job the way it should be done.”
2010, Gordon Lish, "The Friend" in Collected Fictions, ISBN 9780984295050, p. 243 (Google preview):
  â€œ[B]elieve me, she is some catch for the right boy—for a boy which knows which end is up.”
2011, Catherine McGuinness, Emperors' Clothes, ISBN 1937110044, 9781937110048, (Google preview):
  The whole company is in disarray! Nobody knows which end is up.