be with (one)

1. To be following along with and understanding what one is saying. Whoa, slow down, I'm not with you on this. Can you start over from the beginning? Oh yeah, I'm with you—just press these buttons and I'm good to go, right? A: "Are you with me?" B: "I was, until you got to the part about metempsychosis."

2. To share the same views as one. I'm sorry, but I'm with the boss on this one—your plan is just way too risky. I'm with you on sneaking out tonight. Let's do it! What would you do if you found out your boyfriend wasn't with you on politics? And was, like, way on the other side of the political spectrum from you?

3. To attend to a customer. Have a seat, and I'll be with you in just a minute. That woman said she'd be with me soon, but I haven't seen her since! Hi there, I'll be right with you!

4. To be in a romantic relationship with one. She's been with her boyfriend for just six months, but they're already talking about getting married. You know Henry hasn't been with his girlfriend for more than five minutes if he's still gushing about her at every turn. A: "How long have you been with Samantha?" B: "Over a year now. Did you really not know we were together?"

5. slang To have sex with one. I was shocked when he told me how many women he'd been with. I think Sarah was with Keith again last night. Wait, you've been with the girl I've had a crush on for years? But you're my best friend—how could you?

be ˈwith somebody

(informal)
1 understand what somebody is saying or explaining: I’m sorry, but I’m not with you. What exactly did you mean by ‘cut and paste’? ♢ ‘Are you with me?’ ‘No, you’ve lost me. Can you explain it again?’
2 be in support of or agreement with somebody: ‘What do you think, Jonathan?’ ‘I’m with Sarah on this.’
3 used to ask a customer, etc. to wait for a short time: I’m just serving this lady. I’ll be with you in a minute.