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.
      [1913 Webster]

            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):

gratuitous
    adj 1: without cause; "a gratuitous insult"
    2: costing nothing; "complimentary tickets"; "free admission"
       [syn: complimentary, costless, free, gratis(p),
       gratuitous]
    3: unnecessary and unwarranted; "a strikers' tent camp...was
       burned with needless loss of life" [syn: gratuitous,
       needless, uncalled-for]