When you sidle up to your sweetie for a smooch beneath the mistletoe this year, try not to think too much about the provenance of the plant's name. It's hardly the most romantic thing.
The name mistletoe comes from the Old English mistletan (mistel, "mistletoe" or sometimes "basil" + tan, "twig"). Its history before that is unclear, but some etymologists trace the mistel to the German mist or "dung," in which case we're kissing underneath something like the "dung on a twig." This is plausible, given the way the plant is dispersed: Birds eat the plant's berries, but don't digest the seeds. When they drop some little birdy turdies later on, some of the lucky seeds hit a suitable tree branch on the way down to Earth and stick around long enough to grow into something.
Mistletoe is not just one plant, but some 1000-plus loosely related species from all over the world. Many of these are what are called hemi-parasites. The plants have green leaves and perform photosynthesis, but also pilfer some nutrients from a host organism. Once a seed has gone from bird poop to branch and germinated, the mistletoe sends its roots down into the tree's wood to steal some of its water and nutrients. The mistletoe grows on the free food into a thick tangle of stems that Europeans often referred to as "witches’ brooms" and that the Navajo called "baskets on high."
These "baskets" are pretty important to a lot of animals. Birds and bugs feed on the berries and build nests in the stems. Tangles of mistletoe are especially popular as nesting sites for owls, hawks and other raptors. In the winter, when fresh foliage is scarce, elk, deer, moose and even domestic cattle will turn to mistletoe leaves and berries as a high-protein snack. It's not something you'd want to munch on while you wait for the Christmas goose to finish cooking, though—mistletoe is mildly toxic to humans.
This isn't to say we haven't had some use for it over the years. The ancient Druids believed oak trees to be sacred, and accepted any mistletoe that grew on oaks as gifts from the heavens. They would harvest the plant from the trees and decorate their homes with it for the winter solstice. The Druids probably didn't kiss under their mistletoe, though, and no one seems to really know where or how that tradition began. It's been variously attributed to the Greeks, the Norse, the Romans, and the Babylonians.