Is it ensure, insure, or assure?
c.1400, "boundary, limit" (usually as confines), from Old French confins "boundaries," from Medieval Latin confines, from Latin confinium (plural confinia) "boundary, limit," from confine, neuter of confinis "bordering on, having the same boundaries," from com- "with" (see com-) + finis "an end" (see finish (n.)).
1520s, "to border on," from Middle French confiner, from confins (n.); see confine (n.). Sense of "keeping within limits" is from 1590s. Related: Confined; confining.