Definition of "coo"

The male [ring dove] will continue nest-coos for 3–4 days until his female partner begins to nest-coo. At that point the male's nest-coo begins to become less frequent […].

1979, Mei-Fang Cheng, “Progress and Prospects in Ring Dove Research: A Personal View”, in Jay S[eth] Rosenblatt, Robert A[ubrey] Hinde, Colin Beer, Marie-Claire Busnel, editors, Advances in the Study of Behavior, volume 9, New York, N.Y., London: Academic Press, section III (Hormones and Behavior: Lehrman’s Hypotheses), page 99

An infant has only cries and coos with which to communicate distress and well-being. Adults have many more ways of expressing themselves. However, their expressions of disease and ease can be boiled down to sophisticated cries and coos. A call for help in whatever form is a cry. A sense of well-being however expressed is a coo. Healing in the context of cries and coos can be viewed as the process of resolving the cries and fostering the coos.

2001, Denton L. Roberts, Caddy Roberts-Williams, “What You Need to Know to Be Useful”, in Living as Healer: (Everyone Does Therapy and Should … Know How), Pasadena, Calif.: Hope Publishing House, page 23

No birds, except as birds of paſſage, flew, / No bee was known to hum, no dove to coo.

1763, C[harles] Churchill, The Prophecy of Famine. A Scots Pastoral, 2nd edition, London: Printed for the author, and sold by G. Kearsly, in Ludgate-street, page 15

DUET—MAYOR and MAUD. […] Like a Dove I'll coo and bill, pretty Maud, / I will not coo and bill, Mr. Mayor.

, [John] O'Keeffe, Songs, Duets, and Chorusses, in the New Musical Farce of Peeping Tom of Coventry. As Performed at the Theatre Royal, [London?: s.n.], page 4

But oh! ſhall I, Misfortune's bondman, ſpeak / Of pleaſures and delights, where ſorrows ſhriek! / Can plaintive nightingale, or turte-dove, / When vultures tear them, ſing or coo of love?

1784, Voltaire, “Memoirs of Voltaire. Written by Himself. Part the Third”, in Memoirs of the Life of Voltaire. Written by Himself. Translated from the French, Dublin: Printed for Messrs. Moncrieffe, Walker, Exshaw, Wilson, Jenkin, Burton, White, Byrne, Marchbank, Cash, and Heery, page 176

The black-bird and the speckled thrush / Good-morrow gave from brake and brush; / In answer cooed the cushat dove, / Her notes of peace, and rest, and love.

1810, Walter Scott, “Canto III. The Gathering.”, in The Lady of the Lake; […], Edinburgh: […] [James Ballantyne and Co.] for John Ballantyne and Co.; London: Longman, Hurst, Rees, and Orme, and William Miller, stanza II, page 99

Then did her soft breath stop and she lay still, her eyes yet open and smiling at the blossoms and the doves who sat upon the window-ledge and lowly cooed and cooed.

1896, Frances Hodgson Burnett, “The Doves Sat upon the Window-ledge and Lowly Cooed and Cooed”, in A Lady of Quality: Being a Most Curious, hitherto Unknown History, as Related by Mr. Isaac Bickerstaff but Not Presented to the World of Fashion through the Pages of The Tatler, and Now for the First Time Written Down, New York, N.Y.: Charles Scribner's Sons, page 360

As Norah Jones coos sweet nothings on the soundtrack, the happy couple—played by Paul Rudd and Amy Poehler—canoodle through a Manhattan montage, making pasta for two, swimming through a pile of autumn leaves, and horsing around at a fruit stand.

2014 June 26, A. A Dowd, “Paul Rudd and Amy Poehler spoof rom-com clichés in They Came Together”, in The A.V. Club‎, archived from the original on 27 November 2017

They were too busy cooing over the baby and his parents were too busy cooing over each other.

2013, Nicola Cornick, chapter 14, in One Night with the Laird (Harlequin HQN Historical Romance), Don Mills, Ont.: Harlequin HQN

I stood outside the door for a space, letting "I dare not" wait upon "I would", as Jeeves tells me cats do in adages, then turned the handle softly, pushed – also softly – and, carrying on into the interior, found myself confronted by a girl in housemaid's costume who put a hand to her throat like somebody in a play and leaped several inches in the direction of the ceiling. "Coo!" she said, having returned to terra firma and taken aboard a spot of breath. "You gave me a start, sir!" […] "If you cast an eye on him, you will see that he's asleep now." "Coo! So he is."

1960, P[elham] G[renville] Wodehouse, chapter VII, in Jeeves in the Offing, London: Herbert Jenkins

The last track on each of the three sections is a professional course, where you can customise your bike by changing the tyres and the size of chainwheel. Coo!

1988 November, Sean Kelly, “Professional BMX Simulator [video game review]”, in Teresa Maughan, editor, Your Sinclair‎, number 35, London: Sportscene Specialist Press, archived from the original on 14 May 2016

We want you to come up with a side splitting caption for a picture drawn by the fair hand of those at System 3. If you turn out to be the Funniest "Person", we'll give you a big wopping model of a dinosaur. Coo.

1989 November, “Competitions”, in Jim Douglas, editor, Sinclair User: The Independent Magazine for the Independent User‎, number 92, London: ECC Publications, archived from the original on 21 October 2013

Coo, I've only had four gallons of extra caffeine coffee today so I'm not my usual talking-to-PR-girlies-for-hours-on-end self. But bear with me a mo while I get myself together (audience waits for an age while he searches through his coat for the golden envelope). Here it is! Coo, and the winner is The NewZealand Story.

1990 April, “Crash Readers’ Awards Ceremony”, in Oliver Frey, editor, Crash: ZX Spectrum‎, number 75, [Ludlow, Shropshire]: Newsfield, archived from the original on 25 June 2017