Is it ensure, insure, or assure?
Old English mistiltan, from mistel "mistletoe" (see missel) + tan "twig." Cf. Old Norse mistilteinn, Norwegian misteltein, Danish mistelten. The second element is cognate with Old Saxon and Old Frisian ten, Old Norse teinn, Dutch teen, Old High German zein, Gothic tains "twig." Venerated by the Druids; the custom of hanging it at Christmas and kissing under it is mentioned by Washington Irving.