…to discover that Whitman’s “barbaric yawp” is alive 111,000 times over in cyberspace, and continues to propagate outwards and onwards. Perhaps also worth noting that Google Scholar has only about 100 instances. Take a look also at gada.be‘s array.
And the OED has this for first use:

1824 MACTAGGART Gallovid. Encycl., Yawp, the cry of a sickly bird; or one in distress.

Doc Searls, bless him, has a part of the whole thing laid out for us. Here’s the immediate context:

…
The spotted hawk swoops by and accuses me.
He complains of my gab and my loitering.
I too am not a bit tamed. I too am untranslatable.
I sound my barbaric yawp over the roofs of the world.