Generalissimo was a rank grade in some organizations, a supreme rank that outranks even a general or marshal in military forces. This grade is often taken by a military governor or head of state to express dominance over all other forces.
In the year 2251, Leonard McCoy met a cadet whose military bearing did not impress the young medic. In order to parody the young man's demeanor, McCoy referred to Cadet James T. Kirk as "generalissimo". Annoyed, Cadet Kirk simply suggested the correction to McCoy's facetious mistake. (TOS - Starfleet Academy novel: Cadet Kirk)