The Collaborative International Dictionary of English v.0.48:

Brier \Bri"er\, Briar \Bri"ar\ (br[imac]"[~e]r), n. [OE. brere,
   brer, AS. br[=e]r, br[ae]r; cf. Ir. briar prickle, thorn,
   brier, pin, Gael. preas bush, brier, W. prys, prysg.]
   1. A plant with a slender woody stem bearing stout prickles;
      especially, species of Rosa, Rubus, and Smilax.
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   2. Fig.: Anything sharp or unpleasant to the feelings.
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            The thorns and briers of reproof.     --Cowper.
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   Brier root, the root of the southern Smilax laurifolia
      and Smilax Walteri; -- used for tobacco pipes. See also
      2nd brier.

   Cat brier, Green brier, several species of Smilax
      (Smilax rotundifolia, etc.)

   Sweet brier (Rosa rubiginosa). See Sweetbrier.

   Yellow brier, the Rosa Eglantina.
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