The Collaborative International Dictionary of English v.0.48:

Fatness \Fat"ness\, n.
   1. The quality or state of being fat, plump, or full-fed;
      corpulency; fullness of flesh.
      [1913 Webster]

            Their eyes stand out with fatness.    --Ps. lxxiii.
                                                  7.
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   2. Hence; Richness; fertility; fruitfulness.
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            Rich in the fatness of her plenteous soil. --Rowe.
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   3. That which makes fat or fertile.
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            The clouds drop fatness.              --Philips.
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WordNet (r) 3.0 (2006):

fatness
    n 1: excess bodily weight; "she disliked fatness in herself as
         well as in others" [syn: fatness, fat, blubber,
         avoirdupois] [ant: leanness, spareness, thinness]