The Collaborative International Dictionary of English v.0.48:

Pillar \Pil"lar\, n. [OE. pilerF. pilier, LL. pilare, pilarium,
   pilarius, fr. L. pila a pillar. See Pile a heap.]
   1. The general and popular term for a firm, upright,
      insulated support for a superstructure; a pier, column, or
      post; also, a column or shaft not supporting a
      superstructure, as one erected for a monument or an
      ornament.
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            Jacob set a pillar upon her grave.    --Gen. xxxv.
                                                  20.
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            The place . . . vast and proud,
            Supported by a hundred pillars stood. --Dryden.
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   2. Figuratively, that which resembles such a pillar in
      appearance, character, or office; a supporter or mainstay;
      as, the Pillars of Hercules; a pillar of the state. "You
      are a well-deserving pillar." --Shak.
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            By day a cloud, by night a pillar of fire. --Milton.
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   3. (R. C. Ch.) A portable ornamental column, formerly carried
      before a cardinal, as emblematic of his support to the
      church. [Obs.] --Skelton.
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   4. (Man.) The center of the volta, ring, or manege ground,
      around which a horse turns.
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   From pillar to post, hither and thither; to and fro; from
      one place or predicament to another; backward and forward.
      [Colloq.]

   Pillar saint. See Stylite.

   Pillars of the fauces. See Fauces, 1.
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The Collaborative International Dictionary of English v.0.48:

Post \Post\, n. [AS., fr. L. postis, akin to ponere, positum, to
   place. See Position, and cf. 4th Post.]
   1. A piece of timber, metal, or other solid substance, fixed,
      or to be fixed, firmly in an upright position, especially
      when intended as a stay or support to something else; a
      pillar; as, a hitching post; a fence post; the posts of a
      house.
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            They shall take of the blood, and strike it on the
            two side posts and on the upper doorpost of the
            houses.                               --Ex. xii. 7.
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            Then by main force pulled up, and on his shoulders
            bore,
            The gates of Azza, post and massy bar. --Milton.
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            Unto his order he was a noble post.   --Chaucer.
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   Note: Post, in the sense of an upright timber or strut, is
         used in composition, in such words as king-post,
         queen-post, crown-post, gatepost, etc.
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   2. The doorpost of a victualer's shop or inn, on which were
      chalked the scores of customers; hence, a score; a debt.
      [Obs.]
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            When God sends coin
            I will discharge your post.           --S. Rowlands.
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   From pillar to post. See under Pillar.

   Knight of the post. See under Knight.

   Post hanger (Mach.), a bearing for a revolving shaft,
      adapted to be fastened to a post.

   Post hole, a hole in the ground to set the foot of a post
      in.

   Post mill, a form of windmill so constructed that the whole
      fabric rests on a vertical axis firmly fastened to the
      ground, and capable of being turned as the direction of
      the wind varies.

   Post and stall (Coal Mining), a mode of working in which
      pillars of coal are left to support the roof of the mine.
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WordNet (r) 3.0 (2006):

from pillar to post
    adv 1: from one place or situation to another; "we were driven
           from pillar to post" [syn: from pillar to post, hither
           and thither]