The Collaborative International Dictionary of English v.0.48:

Unsparing \Un*spar"ing\, a. [Pref. un- not + sparing, p. pr. of
   spare.]
   [1913 Webster]
   1. Not sparing; not parsimonious; liberal; profuse. --Burke.
      [1913 Webster]

   2. Not merciful or forgiving. [R.] --Milton.
      [1913 Webster] -- Un*spar"ing*ly, adv. --
      Un*spar"ing*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unsparingly
    adv 1: in a scathing and unsparing manner; "she criticized him
           scathingly" [syn: scathingly, unsparingly]