Is it ensure, insure, or assure?
Old English cnyttan "to tie with a knot, bind, fasten," related to Old Norse knytja "bind together," Middle Low German knütten "to tie, knot," Old English cnotta "a knot," from Proto-Germanic *knuttjan, from stem *knutt-. Of brows, late 14c. Meaning "to do knitting" (especially plain stitch) is from 1520s. Related: Knitted; knitting.