1. Expression similar to "Oh, gosh".
2. Expression meaning "Oh, gosh, I'm a hick". If you can use "boy howdy" in a sentence with a straight face, you're about as far out of the urban mainstream as I can imagine.
1. Boy howdy, those sure are a lot of apples.
2. Boy howdy, I'm goin' to the big city. Topeka, here I come!