Green’s Dictionary of Slang

1. to understand a person’s line of thought, to follow their reasoning, e.g. I’m definitely with you on that.

2002

Guardian 13 Oct. 🌐 There were no differences between Britain and the Continent, he asserted. New Labour would go the way of the defeated centre-Left governments in Europe unless it got tough on crime. David Blunkett was with him on that.

2. (US black) to have sexual intercourse with.

2008

‘Grandmaster Flash’ Adventures 114: They could be girls you'd only been with once or they could be the mother of your kids.