Green’s Dictionary of Slang

before one can say ‘Jump Nigger’ phr.

(US) very quickly.

1929		E. Caldwell Bastard (1963) 75: I’m a son of a gun if it wasn’t licked off the deck before you could have said ‘Jump nigger!’.
1942		L. Bennett ‘A’merica’ in Jam. Dialect Poems (1949) 46: Be de time yuh sey ‘puss-pickney’ / As yuh y’eye quint, as yuh yawn.