Green’s Dictionary of Slang

1. some form of poison.

1903

J. London People of the Abyss 97: The incurables and the obstreperous were given a dose of ‘black jack’ or the ‘white potion,’ and sent over the divide.

2. (US tramp) a purgative.

1931

Irwin Amer. Tramp and Und. Sl. 29: Black Jack.— [...] any strong purge like jalap.