The term used for when a dude pretends to be a chick online for the sake of sexual arousal, fantasy and gratification purposes. The dude then gets so caught up in his fantasy that he forgets he is a dude and sets up to meet the object of his online affection thereby creating a "Ding Dong Dude" situation.
Guy #1: These last couple weeks have been amazing! I can't wait to meet you in real life!
Guy #2 (pretending to be a girl): Oh my god! I know right?! I'm so excited! I just know we will have even more fun than we have online!
-2 weeks later-
Guy #3: Hey whatever happened to that hottie
you were talking to online? I thought you drove out to meet her or something.
Guy #1: Yeah. I don't really wanna talk about it.
Guy #3: Dude, stop being a pussy! Was she a beast? Hideous? Did she Catfish
Guy #1: No. It was a Ding Dong Dude kinda thing. Like...showed up to meet and it was like whoa...you're a dude.
Guy #3: Whoa. My condolences man.