Is it ensure, insure, or assure?
c.1400, "tube for projectiles," from Anglo-French canon, Old French canon (14c.), from Italian cannone "large tube, barrel," augmentative of Latin canna "reed, tube" (see cane (n.)). Meaning "large ordnance piece," the main modern sense, is from 1520s. Spelling not differentiated from canon till c.1800. Cannon fodder (1891) translates German kanonenfutter (cf. Shakespeare's food for powder in "I Hen. IV").