From the Christmas Carol Songbook:
cantorion.org/ ... tmas-Carol-Songbook-Voice-SATB
"What Child Is This?" is a Christmas carol whose lyrics were written by William Chatterton Dix, in 1865. At the time of composing the carol, Dix worked as an insurance company manager and had been struck by a severe illness. While recovering, he underwent a spiritual renewal that led him to write several hymns, including lyrics to this carol that was subsequently set to the tune of "Greensleeves", a traditional English folk song. Although it was written in Great Britain, the carol is more popular in the United States than in its country of origin today.
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"Greensleeves" is a traditional English folk song. A broadside ballad by the name "A Newe Northen Dittye of ye Ladye Greene Sleves" was registered by Richard Jones at the London Stationer's Company in September 1580, and the tune is found in several late-16th-century and early-17th-century sources, such as Ballet's MS Lute Book and Het Luitboek van Thysius, as well as various manuscripts preserved in the Seeley Historical Library in the University of Cambridge.
The above text from the Wikipedia article "Greensleeves" text is available under CC BY-SA 3.0.