Is it ensure, insure, or assure?
early 15c. (mid-13c. as a surname), "one who buys and sells in gross," from Anglo-French grosser, Old French grossier, from Medieval Latin grossarius "wholesaler," literally "dealer in quantity," from Late Latin grossus "coarse (of food), great, gross" (see gross). Sense of "a merchant selling individual items of food" is 16c.