The Collaborative International Dictionary of English v.0.48:

Lessen \Less"en\ (l[e^]s"'n), v. t. [imp. & p. p. Lessened
   (-'nd); p. pr. & vb. n. Lessening.] [From Less, a.]
   To make less; to reduce; to make smaller, or fewer; to
   diminish; to lower; to degrade; as, to lessen a kingdom, or a
   population; to lessen speed, rank, fortune.
   [1913 Webster]

         Charity . . . shall lessen his punishment. --Calamy.
   [1913 Webster]

         St. Paul chose to magnify his office when ill men
         conspired to lessen it.                  --Atterbury.

   Syn: To diminish; reduce; abate; decrease; lower; impair;
        weaken; degrade.
        [1913 Webster]

WordNet (r) 3.0 (2006):

lessened
    adj 1: impaired by diminution [syn: diminished, lessened,
           vitiated, weakened]
    2: decreased in severity; made less harsh