The Collaborative International Dictionary of English v.0.48:

Complain \Com*plain"\ (k[o^]m*pl[=a]n"), v. i. [imp. & p. p.
   Complained (k[o^]m*pl[=a]nd"); p. pr. & vb. n.
   Complaining.] [F. complaindre, LL. complangere; com- + L.
   plangere to strike, beat, to beat the breast or head as a
   sign of grief, to lament. See Plaint.]
   1. To give utterance to expression of grief, pain, censure,
      regret. etc.; to lament; to murmur; to find fault; --
      commonly used with of. Also, to creak or squeak, as a
      timber or wheel.
      [1913 Webster]

            O loss of sight, of thee I most complain! --Milton.
      [1913 Webster]

   2. To make a formal accusation; to make a charge.
      [1913 Webster]

            Now, Master Shallow, you'll complain of me to the
            king?                                 --Shak.

   Syn: To repine; grumble; deplore; bewail; grieve; mourn;
        regret; murmur.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

complaining \complaining\ (k[o^]m*pl[=a]n"[i^]mg) adj.
   uttering complaints. Opposite of uncomplaining.
   [prenominal]

   Note: [Narrower terms: faultfinding, grumbling(prenominal):
         fretful, querulous, whiney, whining(prenominal),
         whiny; protesting(prenominal), protestant]

   Syn: complaintive.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

complaining
    adj 1: expressing pain or dissatisfaction of resentment; "a
           complaining boss" [syn: complaining(a), complaintive]
           [ant: uncomplaining]