Is it ensure, insure, or assure?
Old English þeccan "to cover," related to þæc "roof, thatching material," from Proto-Germanic *thakan (cf. Old Saxon thekkian, Old Norse þekja, Old Frisian thekka, Middle Dutch decken, Old High German decchen, German decken "to cover"), from PIE *(s)tog-/*(s)teg- "cover" (see stegosaurus).
Old English þæc "roof, thatch," from the source of thatch (v.). Cf. Old Norse þak, Old Frisian thek, Middle Dutch dak "roof," Old High German dah, German Dach "roof."
A bump or hole in the roadRelated Terms
[1849+; because riders bounce up and down as if they were bowing thanks]