The Collaborative International Dictionary of English v.0.48:

Gratuitous \Gra*tu"i*tous\a. [L. gratuitus, from gratus
   pleasing. See Grate, a., Gratis.]
   1. Given without an equivalent or recompense; conferred
      without valuable consideration; granted without pay, or
      without claim or merit; not required by justice.
      [1913 Webster]

            We mistake the gratuitous blessings of Heaven for
            the fruits of our own industry.       --L'Estrange.
      [1913 Webster]

   2. Not called for by the circumstances; without reason,
      cause, or proof; adopted or asserted without any good
      ground; as, a gratuitous assumption.
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            Acts of gratuitous self-humiliation.  --De Quincye.
      -- Gra*tu"i*tous*ly, adv. -- Gra*tu"i*tous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gratuitously
    adv 1: in an uncalled-for manner; "he insulted us gratuitously"