Source : Webster's Revised Unabridged Dictionary (1913)

Coo \Coo\, v. i. [imp. & p. p. {Cooed}; p. pr. & vb. n.
   {Cooing}.]
   1. To make a low repeated cry or sound, like the
      characteristic note of pigeons or doves.

            The stockdove only through the forest cooes,
            Mournfully hoarse.                    --Thomson.

   2. To show affection; to act in a loving way. See under
      {Bill}, v. i. ``Billing or cooing.'' --Byron.

Source : WordNet®

coo
     n : the sound made by a pigeon

coo
     v 1: speak softly or lovingly; "The mother who held her baby was
          cooing softly"
     2: cry softly, as of pigeons