be down with (something)
1. To be in agreement with, in support of, have interest in, or thoroughly enjoy some subject, activity, or thing. I am so down with math, it's my favorite subject in school! Yeah, I'm down with seeing a movie later this evening.
2. To be on friendly terms with or enjoy the company of somebody. I'm down with Cathy, we just had a small disagreement is all. I'm just not down with John, I think he's kind of a jerk sometimes.
3. To be ill or have caught some disease. (More often expressed as "come down with something.") Marigold had to go home early today. I think she's down with the flu.