be all for something/for doing something

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 all for something/for doing something

believe strongly that something should be done: They’re all for saving money where they can. OPPOSITE: be (dead) set against something/against doing something
See also: all, something