The English folk song "Greensleeves" dates back to the late 1500s. The song is a ballad that tells the story of a man rejected by Lady Greensleeves presumably due to her sexually promiscuous lifestyle.

The chorus of the song begins with:

"Greensleeves was all my joy

Greensleeves was my delight,"

Nearly three centuries later, William Dix, a sick and bedridden insurance company manager, turned to scripture to cope with his illness. During this time of religious reawakening, he would pen several hymns and religiously themed poems, which included the Christmas song "What Child Is This?". The song's lyrics are centered around the perspective of the shepherds as they first visit the baby Jesus and wonder at his significance to humanity.

Although Nix wrote the song in 1865, it would not be published until 1871, during which time it was set to the tune of "Greensleeves."

Here the chorus began with:

"This, this is Christ the King,

Whom shepherds guard and angels sing."

More Info: en.wikipedia.org