intransitive. To get married or become involved in a romantic relationship; to engage in sexual activity. Usually with with.

intransitive. colloquial (originally U.S.). To become romantically or sexually involved with.

Chiefly in passive (or as participial adj.). To commit (oneself) emotionally; spec. to have a sexual relationship with.

colloquial. A love affair, a romance; esp. in to have a thing (witha person).

to have something going (with someone), to have an ‘understanding’ or an affectionate relationship (with someone).