Green’s Dictionary of Slang

(orig. US/Can. milit.) the outset, the beginning.

1919		in K.F. Cowing Dear Folks at Home 48: There have been many calls for the ‘leathernecks’ to act as the shock troops on the ‘jump-off’ of an offensive.
1922		Smith’s Wkly (Sydney) 2 Dec. 18/1: Disaster nearly overtook us right at the jump off.
1960		(con. WWII) G. Sire Deathmakers 215: That’s why I emphasized the issue before we made the jump-off.
1972		(con. 1930s–40s) N. Conway Bloods 48: Maybe you could if your platoon hadn’t been wiped out as you moved to the jump-off.
1979		E. Torres After Hours 7: The MCC is gonna be the jump-off for more than just me.
2005		B. Coleman Rakim Told Me 68: ‘We jumped in the car one afternoon to drive to New York and we heard it on the jump off. That let me know: “All right, Ra, it's getting serious”’.