The Collaborative International Dictionary of English v.0.48:

Slightness \Slight"ness\, n.
   The quality or state of being slight; slenderness;
   feebleness; superficiality; also, formerly, negligence;
   indifference; disregard.
   [1913 Webster]

WordNet (r) 3.0 (2006):

slightness
    n 1: the quality of being unimportant and petty or frivolous
         [syn: pettiness, triviality, slightness, puniness]
    2: smallness of stature [syn: delicacy, slightness]
    3: the property of an attractively thin person [syn:
       slenderness, slightness, slimness]