Is it ensure, insure, or assure?
late 14c., "servant who has charge of doors and admits people to a chamber, hall, etc.," from Anglo-French usser (12c.), from Old French ussier, from Vulgar Latin ustiarius "doorkeeper," from Latin ostiarius "door-keeper," from ostium "door, entrance," related to os "mouth." Fem. form usherette is attested from 1925.
"conduct, escort," 1590s, from usher (n.). Related: Ushered; ushering.