Is it ensure, insure, or assure?
"to set in, put or place in," 1520s, from insert, past participle of Middle English inseren "to set in place, to graft, to introduce (into the mind)" (late 14c.), from Latin inserere "to put in, implant," from in- "in" (see in- (2)) + serere "join together" (see series). Related: Inserted; inserting. The noun meaning "something inserted" is from 1893.