Is it ensure, insure, or assure?
Old English ræde, geræde "prepared, ready," of a horse, "ready for riding," from Proto-Germanic *garaidijaz "arranged" (cf. Old Frisian rede "ready," Middle Dutch gereit, Old High German reiti, Middle High German bereite, German bereit, Old Norse greiðr "ready, plain," Gothic garaiþs "ordered, arranged"), from PIE root *reidh- "to ride" (see ride (v.)). Lengthened in Middle English by change of ending. Ready-made first attested early 15c.; ready-to-wear is from 1890.
early 13c., "to administer;" c.1300, "to take aim;" mid-14c., "to prepare, make ready," from ready (adj.). Related: Readied; readying.