Green’s Dictionary of Slang

1. a newly arrived Jewish immigrant.

1931

(con. 1914) C.W. Willemse Behind The Green Lights 298: Morris Kaplan alias the Mock, a ‘mock’ in the slang of the East Side meaning a newly arrived immigrant.

2. a Jew.

1927		H. Miller Moloch 133: We’ve got a big mock for a landlord and he wants to throw us out.
1967		L. Bruce Essential Lenny Bruce 22: I tell all these Mochs — Jewish people, you know.