It meant that you were involved in something seedy, or had inside information on something. Years ago (and the number of years is unclear to me) being 'down with something' had a different meaning on the street, or amongst those who were less. I thought the speaker was confused, at first, until I heard it repeated in different contexts and came to understand that 'up for that' and 'down for that' have approximately the same definition to most people. It shocked me the first time I heard it (sometime in 2013, if I remember correctly). It is common in chat rooms and other online social hangouts, but does not often appear in writing. Stating that you are 'down for that' appears to be a relatively recent phrase coming from slang. If you are addressing someone directly who is presently seated, you could use a command: up with you! (stand up.) (In case it's not clear, 'I am up to something' means 'I have an ulterior motive I don't wish to reveal.') Although you could say I am up to something, you sneaky little devil, you. Or: up with ice cream! (Yay! Ice Cream!) You would never say I am up with something. up with the king! - meaning hey, that king is pretty great. To be up with something is only used in the imperitive, eg.

You could also speak of having a disease, sometimes preceded by the verb to come if the sickness hasn't yet fully taken hold. I'm down with science means 'I am familiar with science' or 'science is a good thing.' To be down with something is a slang phrase, and not terribly common in formal speech or writing. If you are down with something it means that you have knowledge of something or are in agreement with it.

Downy meaning