How did mistletoe become associated with a Christmastime kiss?

The plant is a parasite that can be poisonous. That sounds more sinister than sensual.

Before the legend got lovey-dovey, mistletoe had long been a symbol of good luck. In Virgil’s “Aeneid,” for example, the hero Aeneas brings an evergreen bough with white berries (thought to be mistletoe) with him in order to enter to the underworld, where it remains alive even in winter.

Roman historian Pliny the Elder wrote about how mistletoe was used to fatten cows and how its berries could be made into a juice that is useful in catching birds. Pliny also wrote that the Druids, Celtic people in the British Isles who were lawyers, poets doctors and clergy, “held nothing more sacred than the mistletoe and the tree that bears it.” Mistletoe was used in Druid religious rituals. “It is the belief with them that the mistletoe,” Pliny wrote, “taken in drink, will impart fecundity to all animals that are barren, and that it is an antidote for all poisons.”

Do not, however, try this at home to cure your holiday hangover. The berries on some versions of the plant are poisonous. Mistletoe is still used medicinally — as an alternative treatment for cancer — but it’s more common in Europe. (The U.S. Food and Drug Administration hasn’t approved mistletoe for treatment of cancer or other conditions.)

The sprig’s association with love goes back to the Norse myth of the god Baldur, whose mother Frigga was the goddess of love and marriage. Baldur was supposed to be immune to all living things and thereby invincible, but she forgot about the mistletoe, which another god used to kill Baldur. “In some versions of the tale,” Christopher Beam writes in Slate, “Frigga’s tears then turned into mistletoe berries, which brought Baldur back to life, prompting Frigga to declare mistletoe a symbol of love.”

However, the tradition of kissing under mistletoe started in 19th-century Britain. It even shows up in Charles Dickens’s “The Pickwick Papers,” which includes a scene where women tried to resist being kissed under the mistletoe, but Mr. Pickwick ends up snagging quite a few snogs. “In this context,” Beam writes, “mistletoe was supposed to bring luck to two people who kissed underneath it and bad luck to those who didn’t.”

The greenery need not be associated with romance. When I called Johnson’s Florist and Garden Centers in Georgetown, a clerk noted that she had mostly seen kids urging their parents to buy mistletoe this season — perhaps seeing it a symbol of familial love, or just as a holiday decoration.

After a tough year, we could all use some good luck, couldn’t we? As of Thursday, Johnson’s had sold all their mistletoe.