The Collaborative International Dictionary of English v.0.48:

Muse \Muse\, v. i. [imp. & p. p. Mused; p. pr. & vb. n.
   Musing.] [F. muser to loiter or trifle, orig., to stand
   with open mouth, fr. LL. musus, morsus, muzzle, snout, fr. L.
   morsus a biting, bite, fr. mordere to bite. See Morsel, and
   cf. Amuse, Muzzle, n.]
   1. To think closely; to study in silence; to meditate.
      "Thereon mused he." --Chaucer.
      [1913 Webster]

            He mused upon some dangerous plot.    --Sir P.
                                                  Sidney.
      [1913 Webster]

   2. To be absent in mind; to be so occupied in study or
      contemplation as not to observe passing scenes or things
      present; to be in a brown study. --Daniel.
      [1913 Webster]

   3. To wonder. [Obs.] --Spenser. --B. Jonson.
      [1913 Webster]

   Syn: To consider; meditate; ruminate. See Ponder.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

musing \musing\ adj.
   Thinking long and intensely.

   Syn: brooding, broody, contemplative, meditative, pensive,
        pondering, reflective, ruminative.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

musing
    adj 1: deeply or seriously thoughtful; "Byron lives on not only
           in his poetry, but also in his creation of the 'Byronic
           hero' - the persona of a brooding melancholy young man";
           [syn: brooding, broody, contemplative,
           meditative, musing, pensive, pondering,
           reflective, ruminative]
    n 1: a calm, lengthy, intent consideration [syn:
         contemplation, reflection, reflexion, rumination,
         musing, thoughtfulness]