Is it ensure, insure, or assure?
Old English þider "to or toward that place," altered (by influence of its opposite hider) from earlier þæder "to that place," from Proto-Germanic *thadra- (cf. Old Norse þaðra "there"), from *tha (see that) + PIE suffix denoting motion toward (cf. Gothic -dre, Sanskrit -tra). The medial -th- developed in Middle English but was rare before early 16c. (cf. gather, murder, burden).