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.
Tits or gtfo. Dun dun.
someone who dun-dun dem all
dey call her dun-dun...dun-dun em all!!!
A mexician whine. Comes from "uh uh", but in spanish, and annoying.
Gordito: Hey, make me some tacos.
Jon: Dun dun!