A Danny originally started as an insult where you leave a conversation without explanation, this then quickly evolved into a general insult which is considered to be worse than any other insult ever imagined.
Guy 1: "Wanna play CoD?"
Guy 2: *leaves conversation*
Guy 3: "Dude, we just got proffesionally Danny'd."
Guy 1: "Coming out for a drink tonight?"
Guy 2: "Can't, going over my girlfriends."
Guy 3: "Oh your such a Danny."