Is it ensure, insure, or assure?
c.1300, "to administer justice;" late 14c., "to show (something) to be just or right," from Old French justifiier "submit to court proceedings" (12c.), from Latin iustificare "act justly toward, make just," from iustificus "dealing justly, righteous," from iustus "just" (see just (adj.)) + root of facere "to do" (see factitious). Of circumstances, "to afford justification," from 1630s. Meaning "to make exact" (now largely restricted to typesetting) is from 1550s. Related: Justified; justifying.