Dun Dun
"Dun Dun" is the typed version of the two-note gavel sting from NBC's Law & Order, used online as shorthand for a mock-dramatic reply. The phrase broke out on 4chan's /b/ board in May 2010 after moderators changed the site's post-confirmation screen to read "DUN DUN Post Successful," and users started ending posts with it.
Overview
"Dun Dun" is the all-caps text stand-in for the two-note gavel sting heard between scenes on NBC's Law & Order3. People type it after a mildly dramatic statement to give the moment a mock-courtroom beat, usually as "DUN DUN" or stretched to "DUN DUN DUN" for extra weight4.
Law & Order ran for 20 seasons on NBC before its finale in May 20102. The typed version works as a reaction shorthand along the same lines as "record scratch" or "gasp," hitting hardest after a small revelation or an awkward admission4.
How It Spread
How to Use This Meme
People typically add "DUN DUN" or "DUN DUN DUN" at the end of a message where the tone is mock-serious, often right after a small twist, a petty grievance, or a mildly incriminating detail4. The all-caps form mirrors how loud the Law & Order sting hits on the show, which ran the cue between nearly every scene change across its 20-season run2.