A word used by gamers to indicate a button layout on an Xbox 360/Xbox controller. In this layout, the primary button (the shoot, punch, or accelerate button) is moved to the "A" key on the controller. Most gamers play this green "A" button with their thumb which started the phrase "green thumb"
It is a approved layout on the halo franchise.
Not to be mistaken for the definition for Green Thumb which means a person with a knack for gardenining.
"after playing all weekend, the right trigger on my controller broke. but i used my 1337h4x skills to customize the button layout. I changed the shoot button to "A" and now I've been using the green thumb since.