Source : Webster's Revised Unabridged Dictionary (1913)

Fuller \Full"er\, n. [AS. fullere, fr. L. fullo. See {Full}, v.
   t.]
   One whose occupation is to full cloth.

   {Fuller's earth}, a variety of clay, used in scouring and
      cleansing cloth, to imbibe grease.

   {Fuller's herb} (Bot.), the soapwort ({Saponaria
      officinalis}), formerly used to remove stains from cloth.
      

   {Fuller's thistle or weed} (Bot.), the teasel ({Dipsacus
      fullonum}) whose burs are used by fullers in dressing
      cloth. See {Teasel}.

Source : WordNet®

fuller's earth
     n : an absorbent soil resembling clay; used in fulling
         (shrinking and thickening) woolen cloth and as an
         adsorbent