Is it ensure, insure, or assure?
mid-15c., apparently from Middle English noun blabbe "one who does not control his tongue" (late 13c.), probably echoic. Related: Blabbed; blabbing. The exact relationship between the blabs and blabber is difficult to determine. The noun was "[e]xceedingly common in 16th and 17th c.; unusual in literature since c 1750" [OED].