Originally Posted by foxnine
Rumor has it that DH stands for "Dulles Hub".
Yup, that's the rumor and it dates from the ACA days. In fact, I think it's more than a rumor; in training they told us that's how the code was picked.
If the OP wants to "pick on" FlyI, how about these other codes floating around the domestic US system?
OO, B6, WN, F9, XJ, 9E, ZW, to start with a few...