Green’s Dictionary of Slang

a co-respondent in a divorce case.

1908		Sun. Times (Perth) 11 Oct. 1/1: A divorce that wasn’t is the latest in Leaderville [...] neither party was aware of anything till the private detective chatted the co-re.
1914		Bulletin (Sydney) 5 Nov. 11/4: The defendant admitted to ‘shaping up’ to a man in Collin-street, but pleaded that the threatened party had bolted with his wife. [...] No legislation will ever wholly kill the ancient rite of ‘jobbing’ the co.-re.
1942		C. Burt Case of the Fast Young Lady in DSUE (1984).