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coo (v.)

1660s, "to utter a low, plaintive, murmuring sound," echoic of doves. Compare, in the same sense, Danish kurre, German girren; also Hindi kuku "the cooing of a dove," Persian huhu "a dove," and see cuckoo.

Meaning "to utter by cooing" is from 1798. Meaning "to converse affectionately, make love in murmuring endearments" is from 1816. Related: Cooing. The noun is recorded from 1729.

What are you doing now,
   Oh Thomas Moore?
What are you doing now,
   Oh Thomas Moore?
Sighing or suing now,
Rhyming or wooing now,
Billing or cooing now,
   Which, Thomas Moore? 
[Lord Byron, from "To Thomas Moore," 1816]