mistletoe (n.)

European plant growing parasitically on certain trees, Old English mistiltan, from mistel "mistletoe" (see missel) + tan "twig," from Proto-Germanic *tainan "twig" (source also of Old Saxon and Old Frisian ten, Old Norse teinn, Dutch teen, Old High German zein, Gothic tains "twig"). Similar formation in Old Norse mistilteinn, Norwegian misteltein, Danish mistelten.

Venerated by the Druids, especially when found growing on the oak, which it seldom does; the custom of hanging it at Christmas and kissing under it is mentioned by Washington Irving. The alteration of the ending according to Century Dictionary is perhaps from a mistaking of the final -n for a plural suffix after tan fell from use as a separate word, but OED finds it a natural evolution in West Saxon based on stress.