Delusions of grandeur
A false and exaggerated belief about one's status or importance.
Delusions of grandeur are often considered to be synonymous with megalomania. While the two conditions are somewhat similar, psychiatrists might make the distinction that megalomania - a self-important preoccupation with power and control - isn't necessarily delusional. Fascist dictators like Hitler show clear signs of megalomania, but may not be (in that sense at least) delusional.
Who the first person to display delusions of grandeur was we are never likely to know. The term itself entered the language in the late 19th century and the first person who is recorded as being publicly accused of the condition was the prominent New York tailor Henry Prouse Cooper.
Mr. Cooper was subject to a judicial enquiry for insanity in 1882, an investigation brought about by his brother Stephen, who was also a business partner. H. P. Cooper made no defence against the charge of insanity, in fact he loudly proclaimed it and insisted on being taken to an asylum - "Take me away at once. Don't you see I am a slave to women and rum!". The 'delusions' that his brother was concerned about included a false claim to have opened a successful department store in Paris and wildly optimistic plans for bizarre property developments in New York. In the course of the enquiry, Mr. Cooper was described as having 'delusions of grandeur'. Despite the evidence (which we might today easily see as being consistent with bipolar disorder and alcoholism), H. P. Cooper was judged sane and went about his business.