"Yeah, I'm down for that", he replied.
Translation into simple-English:
"Do you want to do that?"
"Yes, I do want to do that."
After the matter was settled (he "did want to do that"), I noted with interest our use of opposite words ("up" and "down") to mean the same. How could this have come about? A linguistic puzzle, perhaps.
"Down for that" is a phrase I have never used. It's always sounded "street" to me, by which I mean not-far-removed from ebonics. If I used "down for that", I might as well call people "homeys" and so on. I'm not a "whigger" (as Whites who ostentatiously imitated Black speech were called, disparagingly, in my school-days). On the other hand, phrases / words from Black-slang have frequently shifted into general (White) usage in the past century or two, haven't they.
That now-ubiquitous word "cool" is one of the words that comes from Black slang, so they say. When did it become "non-racial" and become commonly used by (non-"whigger") Whites? I don't know. 1970s? Another linguistic puzzle.
What seems obvious to me, anyway, is that there have thus always been people alive who'd developed their linguistic-sensibilities before the crossover occurred. White men of my grandfathers' generation (both of mine were born in the 1910s-USA) would never have used the word "cool" unless the temperature were involved. Maybe I'm just a bit too old, too, and this "down for that" has been, unbeknownst to me, becoming "mainstream" these past years, after I'd already cast my lot against it. / I can't imagine ever bringing myself to use it; I'd feel like a true idiot.