Is it ensure, insure, or assure?
late 14c., "rope for fastening an animal," probably from Old Norse tjoðr "tether," from Proto-Germanic *teudran (cf. Danish tøir, Swedish tjuder, Old Frisian tiader, Middle Dutch tuder, Dutch tuier "line, rope," Old High German zeotar "pole of a cart"), from PIE root *deu- "to fasten" + instrumentive suffix *-tro-. Figurative sense of "measure of one's limitations" is attested from 1570s.
late 15c., from tether (n.). Related: Tethered; tethering.