NOUN (1)

1. the sound made by a pigeon;


VERB (2)

1. speak softly or lovingly;
- Example: "The mother who held her baby was cooing softly"

2. cry softly, as of pigeons;

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            The stockdove only through the forest cooes,
            Mournfully hoarse.                    --Thomson.
      [1913 Webster]

   2. To show affection; to act in a loving way. See under
      Bill, v. i. "Billing or cooing." --Byron.
      [1913 Webster] Cooey

WordNet (r) 3.0 (2006):

coo
    n 1: the sound made by a pigeon
    v 1: speak softly or lovingly; "The mother who held her baby was
         cooing softly"
    2: cry softly, as of pigeons