The Collaborative International Dictionary of English v.0.48:

Oxalic \Ox*al"ic\, a. [From Oxalis: cf. F. oxalique.] (Chem.)
   Pertaining to, derived from, or contained in, sorrel, or
   oxalis; specifically, designating an acid found in, and
   characteristic of, oxalis, and also certain plant of the
   Buckwheat family.
   [1913 Webster]

   Oxalic acid (Chem.), a dibasic acid (HO.CO.CO.OH),
      existing combined in oxalis as an acid potassium oxalate,
      and in many plant tissues as the calcium oxalate. It is
      prepared on a large scale, by the action of fused caustic
      soda or potash on sawdust, as a white crystalline
      substance, which has a strong acid taste, and is poisonous
      in large doses. It is used in dyeing, calico printing,
      bleaching flax and straw, the preparation of formic acid,
      and in salts of lemon for removing ink stains, mold, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oxalic acid
    n 1: a toxic colorless crystalline organic acid found in oxalis
         and other plants; used as a bleach and rust remover and in
         chemical analysis [syn: oxalic acid, ethanedioic acid]