The use of mistletoe dates back to pagan times of course, just as the Christmas tree, advent wreaths, and the use of other greenery does. It is actually a hemiparasite plant that grows upon tree trunks. It was once considered an aphrodisiac by the ancients and associated with fertility by the ancient Celts and Germans. The idea of kissing under the mistletoe originated with Saturnalia, the ancient pagan feast, and later extended into pagan marriage rites.
The custom of kissing under the mistletoe continues into modern times, although some have taken advantage of the practice and landed in jail.