Source : Webster's Revised Unabridged Dictionary (1913)

Pensiveness \Pen"sive*ness\, n.
   The state of being pensive; serious thoughtfulness;
   seriousness. --Hooker.

Source : WordNet®

pensiveness
     n 1: persistent morbid meditation on a problem [syn: {brooding}]
     2: deep serious thoughtfulness [syn: {meditativeness}, {contemplativeness}]