Like other evergreens, the mistletoe is a symbol of immortality. Since it thrives in the trees rather than being rooted in the earth, it is considered representative of both the divine and the topsy-turvy. There's my Gob reference.
Now get this one, parasitic plants, mistletoe symbolizes the feminine need for masculine protection and provision. Don't send me angry letters, it's an ancient symbol. Its white berries make this plant a lunar, female, or fertility symbol. Diana (a.k.a. Artemis), fertility goddess of the Ephesians wore a crown of mistletoe as an emblem of fertility and immortality (Acts 19:24-41). See!
Today, kissing under the mistletoe is a popular Christmas custom. However, few remember that a berry of the plant is to be plucked off for each kiss. When the sprig runs out of berries, there is to be no more kissing. According to Crippen, any unmarried lady who is not kissed under the Christmas mistletoe will remain single for yet another year. A romantic symbol, this plant represents happiness, peace, and welcome. It is believed that ancient enemies, accidentally meeting under wild mistletoe in the forests, would embrace and enjoy a day of truce. Which is a bit, er, un-masculine to put it delicately.
Kissing may have its origin in the Scandinavian myth of Balder, the Norse equivalent of Apollo the sun god. According to this myth, when Balder was born, his mother, Frigga, goddess of love and beauty, obsessed with concerns for her son's safety, created charms and drew promises from every creature, plant, and object that they would do no harm to him. Unfortunately, Frigga forgot to extract a promise from the mistletoe and the evil god, Loki, tricked his blind brother into throwing a mistletoe spear at Balder and inadvertently killing this god of sunlight and vegetation. The death of Balder brought winter into the world and caused Frigga to cry so pitifully that her tears turned into the plant's white berries. Fortunately, the gods restored Balder to life. Then Frigga declared that the mistletoe must ever after bring love rather that death into the world. Everyone passing under this plant was enjoined to embrace as Frigga planted a kiss of gratitude upon them in memory of the resurrection of her son. This was the worst issue of Thor in the history of Marvel Comics, btw.
Although its pagan associations often caused mistletoe to be banished from Christian festivities, it remained a popular Christmas symbol of love and eternal life. It is parasitic, afterall. It was called Herbe de la Croix and Lignum Sanctae Crucis or "the Wood of the Sacred Cross" because it was believed to have been the tree which supplied the wood for the cross of Christ. Which seems completely insane, since it isn't... oh, wait for it... For its part in the Crucifixion, it was condemned to the life of a parasitic vine, in the same way the serpent was condemned to crawl upon its belly for its part in the fall of man. Some believe that an additional penance was required: that the mistletoe bestow good fortune and blessings upon everyone who walked beneath it. You know - cause plants can do that.
Because it grew on the sacred oaks they worshiped, Druids believed mistletoe was of divine origin; brought to earth by a stroke of lightning. It was believed to share in the strength of the oak. After the winter solstice, they gathered the plant with great ceremony in a sacred fertility ritual. It was cut with a golden sickle and caught in a white cloth to avoid its contamination by contact with iron or the earth. It was then offered to their gods along with a sacrificial bull. But why would they put it on their weapons? And wouldn't the lightning destroy the tree?
With the coming of the new year, mistletoe becomes the legendary Golden Bough. Its withered yellow leaves were believed to assist its owner in the search for buried treasure. According to Virgil's story, Aeneas, leader of the Trojan refugees, carried the Golden Bough into the Underworld to seek news of his future from his deceased father. Other legends state that slaves might win their freedom by touching the sacred Golden Bough. But that's another story.
Unless otherwise indicated, all scripture quotes are from the NKJV Bible.
Sources almost entirely lifted from ChristStory Mistletoe Page.