Geronimo!, excl. — Green’s Dictionary of Slang

Geronimo! excl.

[proper name Geronimo, nickname of Apache leader, Goyathlay (‘One Who Yawns’) (1829–1909); note NYHT 19/5/1941: ‘The use of “Geronimo” dates back to the early days of the 501st Parachute Battalion, ’way back in last October. Two sergeants got into an argument about being afraid, when the first left the plane. One said that to prove he was not scared stiff he would yell something as he jumped. When he left the plane the only thing that came to mind was the name of the famous Indian chief. So he hollered out “Geronimo!”’]

(US, orig. army) a cry made when leaping or about to start a fight.

1941		N.Y. Herald Trib. 9 May 19/1: When a parachutist steps to the open door of a plane [...] and then catapults his body out into the air, he invariably shouts ‘Geronimo!’ If there are twelve men making a mass jump they all yell ‘Geronimo!’ They shout it with such vehemence that those watching from the ground can hear it distinctly.
1955		H. Kurtzman Inside Mad (2002) 143: Hoo ha Geronimo!
1965		W.C. Anderson Adam M-1 185: He roared, ‘Geronimo!’.
1970		C. Brown Down All the Days 33: ‘The dirty lousy bastards – hitting a cripple! Geronimo! ...’ And off they flew in maddened pursuit.
1983		A-Team Storybook 7: With a cry of ‘Geronimo!’ Murdock jumped down.
1994		F. Mac Anna Ship Inspector 210: Mao leaned over my shoulder and roared ‘Ger-on-i-mo’ at the top of his voice.