Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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