Green’s Dictionary of Slang

(UK society) a co-respondent (in a divorce case); in weaker sense a lover or mistress.

1890		Sporting Times 4 Jan. 1: Mr. Parnell was made a ‘co’ and a charge of adultery is no longer looked upon as a matter of any consequence as affecting political position.
1905		Sun. Times (Perth) 8 Oct. 4/7: Should a pretty little madam go to Court and fail to show / That her hubby had relations with a deep designing co.
1907		Sun. Times (Perth) 11 Aug. 4s/7: ‘Paternal care‘ you’ll not bestow / On madams who a-going go, / Or you’ll be cited as the co.
1910		Truth (Perth) 13 Aug. 4/6: That's where married wimmen linger / Waiting for to meet the ‘Co’ / While their husbands are a-workin’ / Or a-boozin’ don’tcherknow.