The Collaborative International Dictionary of English v.0.48:

Fucus \Fu"cus\, n.; pl. Fuci. [L. rock lichen, orchil, used as
   a red dye, red or purple color, disguise, deceit.]
   1. A paint; a dye; also, false show. [Obs.]
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   2. (Bot.) A genus of tough, leathery seaweeds, usually of a
      dull brownish green color; rockweed.
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   Note: Formerly most marine alg[ae] were called fuci.
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