I have done a little research on your behalf, and it turns out that the giant may much less ancient than a lot of people assume.
As Stones of England says:
The first reference to this figure dates back to 1694: a payment in the Cerne Abbas churchwarden's accounts of 3 shillings towards the re-cutting of the giant. The first written reference is by John Hutchins in his Guide to Dorset, 1751, but no one knows exactly when or who first cut the Giant.
Recently, the historian Ronald Hutton stated that it was cut in the 17th century by the Lord Holles' servants. In fact, it's unusual that, unlike the Uffington White Horse, there is no reference to the Cerne Abbas Giant in Medieval documents.
During the Civil War (1644 - 1660), Lord Holles was Lord of the Manor but his estate was sequestered and mismanaged by his steward. Maybe then his servants, in this period of chaos, cut the giant in the hillside.