Green’s Dictionary of Slang

(US) a cry of encouragement.

1889–90

Barrère & Leland Dict. of Sl., Jargon and Cant I 507/1: On the New York Stock Exchange, says Medbury in ‘Men and Mysteries of Wall Street’ (cited by Bartlett), if a member drops asleep, ‘Josh! josh!’ comes roaring from a dozen lungs, and the broker is awakened by the cry.