Green’s Dictionary of Slang

j.c.o. n.

[just come over]

(US) a newly arrived immigrant.

[US](con. 1940s) G. Pelecanos Big Blowdown (1999) 57: ‘You and all the JCOs, talkin’ away. That must of been a sight.’ ‘JCOs? [...] They didn’t “just come over”. Hell, Joe, they been here over twenty years.’.