Howdy is a friendly way to say hello. You hear it most in the United States, especially in the South and the West — cowboy films love it. It is warm and casual; you would not put it in a job letter or say it to a judge. Though actually, people far from Texas now say it too, a little for fun, because it sounds so “American.” In Britain it is not the everyday hello. You can, in old or playful English, use it as a verb (“He howdied us at the gate”), but almost everyone just uses it as a greeting. It is a squeezed-down “How do you do?” — so the tidy idea is a polite question, but in real talk it often means no more than “Hi.”