The sound between scenes in Law and Order. The expression originated when 4chan changed the "Post Successful" page and font to look like the Law and Order black screen between scenes. The page played the sound, "Dun Dun," from Law and Order. 4chan /b/tards began to post the expression, "Dun Dun," anywhere to import a sense of finality or just randomly.

The phenomenon of having to come out as bisexual to the same person/people repeatedly because they assumed or hoped that your sexuality was a "phase." Refers less to the holiday and more to the 1993 Bill Murray comedy film in which the main character experiences the same day over and over.

ur people. ur ace

someone who dun-dun dem all

When a person can't get himself to talk about a certain ongoing subject (normally on chatrooms) and is put in a awkward situation, where some might expect a reaction from him and he goes,

"dun dun."

It comes from "so done with this". Usually related with Otaku culture for some reason.

A mexician whine. Comes from "uh uh", but in spanish, and annoying.

pronounced (doon-doon);

doing the dirty; making babies;
the act of performing inappropriate stuff aka sex

just another way to say sex without saying sex