Ingredient in the Antidote to Most Poisons and the Forgetfulness Potion (Pm). Mistletoe is used as Christmas decoration (PS12, CS12, GF23, OP21, HBP15), and was part of the decorations in the Room of Requirement just before the Christmas holidays Luna said that it keeps away Nargles (OP21). The Lovegoods have mistletoe growing on either side of their front door with a sign saying "PICK YOUR OWN MISTLETOE" (DH20).
In the lore of the druids, mistletoe signified life-force and fertility, hence the tradition of kissing under it.
Mistletoe berries are used in Antidote to Common Poisons and Forgetfulness Potion on Pottermore
Old English "mistiltan" - Germanic "mist" = "dung" + tan "twig" for the way birds spread the seeds tree to tree