The Collaborative International Dictionary of English v.0.48:

Appui \Ap`pui"\, n. [F., fr. L. ad + podium foothold, Gr. ?,
   dim. of ?, ?, foot.]
   1. A support or supporter; a stay; a prop. [Obs.]
      [1913 Webster]

            If a vine be to climb trees that are of any great
            height, there would be stays and appuies set to it.
                                                  --Holland.
      [1913 Webster]

   2. (Man.) The mutual bearing or support of the hand of the
      rider and the mouth of the horse through the bit and
      bridle.
      [Webster 1913 Suppl.]

   Point d'appui. [F., a point of support.] (Mil.)
      (a) A given point or body, upon which troops are formed,
          or by which are marched in line or column.
      (b) An advantageous defensive support, as a castle,
          morass, wood, declivity, etc.
      (c) any point of support or basis of operations, as a
          rallying point.
          [1913 Webster +Webster 1913 Suppl.]