Is it ensure, insure, or assure?
mid-15c., from Latin aspersionem (nominative aspersio) "a sprinkling," noun of action from past participle stem of aspergere "to sprinkle on," from ad- "to" (see ad-) + spargere "sprinkle, strew" (see sparse). Originally in theology, the shedding of Christ's blood. Modern sense of "bespattering with slander" first attested 1590s. To cast aspersions was in Fielding (1749).