Mistletoe is a plant that grows on a variety of trees like apple, willow and oak. The name mistletoe comes from ancient Anglo-Saxons. In Anglo-Saxon “mistel” means “dung” and “tan” means “twig,” which means dung on a twig. This is because the ancient Anglo-Saxons noticed the mistletoe would often grow where birds leave droppings, which is how the name came to be.
The tradition of kissing under the mistletoe started in ancient Greece, during the festival of Saturnalia and later in marriage ceremonies, because of the plant's association with fertility. During the Roman era, enemies at war would reconcile their differences under the mistletoe, which to them represented peace. Romans also decorated their houses and temples with mistletoe in midwinter to please their gods.
There is also a Nordic myth concerning mistletoe, and it goes like this: The plant was sacred to Frigga, the goddess of love, but Loki, commonly known as the god of mischief, shot Frigga's son with a spear or, in some telling’s, an arrow carved from mistletoe. Frigga revived her son under the mistletoe tree and decreed that anyone who stands under the mistletoe tree deserves not only protection from death, but also a kiss.
In Victorian England, kissing under the mistletoe was serious business. If a girl refused a kiss, she shouldn't expect any marriage proposals for at least the next year, and many people would snub their noses at her, remarking that she would most likely end up an old maid.
Today, we take a much more lighthearted approach to the tradition. Although many couples simply just kiss when caught standing under it, there is a proper etiquette dating back to ancient times about kissing under the mistletoe. Linda Allen writes in Decking the Halls: The Folklore and Traditions of Christmas Plants that the gentleman should pluck one white berry while kissing the lady on the cheek. One kiss is allowed for each berry.
It should be mentioned, however, that the plant contains toxic amines, and eating its berries can cause vomiting and stomach pain. In the past, mistletoe had been thought to be a cure for epilepsy and other ailments but was proved false. In fact, mistletoe is probably more harmful than helpful: deaths have even been reported from drinking too much tea made from its berries.