The Collaborative International Dictionary of English v.0.48:

Portcullis \Port*cul"lis\, n. [OF. porte coulisse, cole["i]ce, a
   sliding door, fr. L. colare, colatum, to filter, to strain:
   cf. F. couler to glide. See Port a gate, and cf. Cullis,
   Colander.]
   1. (Fort.) A grating of iron or of timbers pointed with iron,
      hung over the gateway of a fortress, to be let down to
      prevent the entrance of an enemy. "Let the portcullis
      fall." --Sir W. Scott.
      [1913 Webster]

            She . . . the huge portcullis high updrew. --Milton.
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   2. An English coin of the reign of Elizabeth, struck for the
      use of the East India Company; -- so called from its
      bearing the figure of a portcullis on the reverse.
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The Collaborative International Dictionary of English v.0.48:

Portcullis \Port*cul"lis\, v. t. [imp. & p. p. Portcullised;
   p. pr. & vb. n. Portcullising.]
   To obstruct with, or as with, a portcullis; to shut; to bar.
   [R.] --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

portcullis
    n 1: gate consisting of an iron or wooden grating that hangs in
         the entry to a castle or fortified town; can be lowered to
         prevent passage