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

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "coo":
   aspirate, bark, bawl, bellow, blare, blat, blubber, boom, bray,
   breathe, buzz, cackle, call, carol, caw, chant, chatter, cheep,
   chirk, chirp, chirr, chirrup, chitter, chuck, clack, cluck,
   cock-a-doodle-doo, croak, cronk, crow, cuckoo, drawl, drum,
   exclaim, flute, gabble, gaggle, gasp, gobble, growl, grunt, guggle,
   hiss, honk, hoo, hoot, keen, lilt, maffle, mumble, murmur,
   mussitate, mutter, pant, peep, pip, pipe, quack, roar, roll,
   rumble, scold, scream, screech, shriek, sibilate, sigh, sing, snap,
   snarl, snort, sob, squall, squawk, squeal, susurrate, thunder,
   trill, trumpet, twang, tweet, twit, twitter, wail, warble, whine,
   whisper, whistle, yap, yawp, yell, yelp