The Collaborative International Dictionary of English v.0.48:

Thinness \Thin"ness\, n.
   The quality or state of being thin (in any of the senses of
   the word).
   [1913 Webster]

WordNet (r) 3.0 (2006):

thinness
    n 1: relatively small dimension through an object as opposed to
         its length or width; "the tenuity of a hair"; "the thinness
         of a rope" [syn: thinness, tenuity, slenderness]
         [ant: thickness]
    2: the property of having little body fat [syn: leanness,
       thinness, spareness] [ant: avoirdupois, blubber,
       fat, fatness]
    3: the property of being very narrow or thin; "he marvelled at
       the fineness of her hair" [syn: fineness, thinness]
    4: the property of being scanty or scattered; lacking denseness
       [syn: sparseness, spareness, sparsity, thinness]
    5: a consistency of low viscosity; "he disliked the thinness of
       the soup" [ant: thickness]