The Collaborative International Dictionary of English v.0.48:

Buttress \But"tress\, n. [OE. butrasse, boterace, fr. F. bouter
   to push; cf. OF. bouteret (nom. sing. and acc. pl. bouterez)
   buttress. See Butt an end, and cf. Butteris.]
   1. (Arch.) A projecting mass of masonry, used for resisting
      the thrust of an arch, or for ornament and symmetry.
      [1913 Webster]

   Note: When an external projection is used merely to stiffen a
         wall, it is a pier.
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   2. Anything which supports or strengthens. "The ground pillar
      and buttress of the good old cause of nonconformity."
      --South.
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   Flying buttress. See Flying buttress.
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The Collaborative International Dictionary of English v.0.48:

Buttress \But"tress\, v. t. [imp. & p. p. Buttressed (?); p.
   pr. & vb. n. Buttressing.]
   To support with a buttress; to prop; to brace firmly.
   [1913 Webster]

         To set it upright again, and to prop and buttress it up
         for duration.                            --Burke.
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WordNet (r) 3.0 (2006):

buttress
    n 1: a support usually of stone or brick; supports the wall of a
         building [syn: buttress, buttressing]
    v 1: reinforce with a buttress; "Buttress the church"
    2: make stronger or defensible; "buttress your thesis"