я за

Я вижу это только в https://www.urbandictionary.com/define.php?term=to%20be%20down

Стоит сказать, что в других местах - другие значения, в т.ч. и почти противоположные (в определенном контексте), как, например, "to dislike or not approve of someone or something" - https://www.macmillandictionary.com/dictionary/british/be-down-on

https://www.macmillandictionary.com/dictionary/british/be-down-on

But there’s no ON here

The way it’s written here, it only means one thing: I’m game.
A: Who wants to go to the movies?
B: i’m down.

А почему 'down' -- "за", а не 'up'?!

I'm down!" which is the same as "yeah! I'll come!". Or the less common meaning is if someone says "I'm feeling down" which means they are sad.

Ули, скажи