Source : Webster's Revised Unabridged Dictionary (1913)

   {To sit under}, to be under the instruction or ministrations
      of; as, to sit under a preacher; to sit under good
      preaching.

   {To sit up}, to rise from, or refrain from, a recumbent
      posture or from sleep; to sit with the body upright; as,
      to sit up late at night; also, to watch; as, to sit up
      with a sick person. ``He that was dead sat up, and began
      to speak.'' --Luke vii. 15.