be for it

be (all) for (someone or something)

To support or be in favor of a person, group, or effort. If management is considering adding a gym to the building, I'm all for it! I respect your opinion, but I'm for the other candidate. I don't know about the rest of you, but I'm all for going on a hike!

be for it

be in imminent danger of punishment or other trouble. British informal
1997 Peter Carey Jack Maggs The master. He reads to me. He would be reading to me now but I said I was ill and must go back to my bed. I'm for it if he finds me gone.