Is it ensure, insure, or assure?
1714, first used by Joseph Addison; see ego + -ist. Addison credits the term to "Port-Royalists" who used it in reference to obtrusive use of first person singular pronoun in writing, hence "talking too much about oneself." Meaning "self-conceit, selfishness" is 1800. The -t- is abnornmal, perhaps by influence of dogmatism. Related: Egotistic; egotistical.