Is it ensure, insure, or assure?
Old English cyrnel "seed, kernel, pip," from Proto-Germanic *kurnilo- (cf. Middle High German kornel, Middle Dutch cornel), from the root of corn "seed, grain" (see corn (n.1)) + -el, diminutive suffix. Figurative sense of "core or central part of anything" is from 1550s.
(Note: NOT "kernal").