Green’s Dictionary of Slang

having a relationship with.

1849		Thackeray Pendennis I 111: ‘I wonder whether she is on with Sir Derby Oaks,’ thought Bows.
1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 107: On with: in push circles, keeping company on [sic] courting is spoken of as ‘I am on with her’, ‘She is on with me.’.
1905		Sun. Times (Perth) 17 Dec. 1/1: The Highgate Hill Venus is on again with the old love.
1910		L. Esson Woman Tamer in Ballades of Old Bohemia (1980) 67: Wasn’t he on with Katie before?
1959		G. Hamilton Summer Glare 68: All the boys in town are on with her.
1969		A. Buzo Rooted I iii: I’m on with Simmo now. We’re going to make it together.
1977		B. Davidson Collura (1978) 103: Collura was on dead center with Dante Diamonds.
1987		Eble Campus Sl. Oct. 5: on – continuing a relationship: Are you still on with her?
2003		McGill Reed Dict. of N.Z. Sl.
2007		L. Redhead Cherry Pie [ebook] ‘t looks like your husband was on with her’.