It was originally a name (a variant of Jerome). The best known Geronimo was a leader of the Bedonkohe Apache who lived until 1909 (see http://en.wikipedia.org/wiki/Geronimo). During the 20th century, the name was adopted by US parachute troops as a yell when jumping out of an aircraft (see http://www.straightdope.com/columns/read/1119/why-do-parachutists-yell-geronimo). From that, its use broadened into a "battle cry" when taking an irrevocable leap into any hazardous (or merely unknown) situation. It was used in that way by Ace, companion of the Seventh Doctor (in "Battlefield", for example).
See also What language is "Geronimo"?