What does "appui" mean?

Appui: A support or supporter; a stay; a prop. [Obs.] If a be to climb trees that are of any great height, there would be stays and appuies set to it. Holland. Point d'appui. Etym: [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.