Tales of the wicker man come out of a persistent (but arguably false) legend of the ancient Druids. The tale, recounted in Julius Caesar's Gallic wars, recounts a sensationalistic story of human sacrifices made by herding victims into a large man-shaped wicker cages and then burning them:
"Others have effigies of great size interwoven with twigs, the limbs of which are filled up with living people which are set on fire from below, and the people are deprived of life surrounded by flames. It is judged that the punishment of those who participated in theft or brigandage or other crimes are more pleasing to the immortal gods; but when the supplies of this kind fail, they even go so low as to inflict punishment on the innocent "
There are no other contemporary accounts (other than those who drew on Caesar's writings) of this practice, and no archaeological evidence to support the accusation, yet this story is the basis for most of the tales about Druids as bloodthirsty practitioners of human sacrifice.
The truth in the tale is probably related to the ancient harvest custom, still carried out in parts of Europe, of ritually sacrificing the "corn god" to ensure the following year's harvest. A small portion of the grain was left in the field, often twisted or tied into a man-like shape, often called a 'corn dolly.' This effigy (immortalized in Burns' ballad of John Barleycorn) was believed to contain the essence of the spirit of the grains- a representation of the solar deity who would be burned and 'reborn' as the spring grain.
Today's wicker man is largely a Neopagan custom that bears only passing relation to his ancient forebear. A fixture at harvest festivals and Beltane gatherings, he often represents the passing of the old year and its cares and worries.