Green’s Dictionary of Slang

with prep.

(US short order) denoting ‘with’ some form of accompaniment, e.g. chilli, relish .

Morn. Call (Allentown, PA) 24 Feb. 17/2: ‘With’ [...] means add onions, chili, relish and what not.

In phrases

who are you with?

(US und.) a coded introductory question which is used to established which branch of the US Mafia a given individual is linked to.

[US]B. McCarthy Vice Cop 218: ‘“Who are you with?” Just like that. That question is the classic mob introduction. It takes the place of “How do you do?” Then, before I even have a chance to answer, he says, “My name is Angelo. From now on you’re with me. Anybody bothers you, just say you are with me and I am Carmine-by-the-Bay”’.