Desu
Desu is the Japanese copula word です turned 4chan spam meme, forever tied to the Rozen Maiden character Suiseiseki, who ends every sentence with it. It started as ammunition in an April 2006 forum raid between 4chan's /a/ and /b/ boards, and became one of the defining weeaboo-era in-jokes of the late 2000s.
Overview
Desu is the polite Japanese copula, roughly meaning "is" or "to be," normally tacked onto the end of statements in everyday Japanese speech2. Online it turned into a spam word blasted into 4chan threads, usually alongside Suiseiseki, the Rozen Maiden doll character mentioned in nearly every desu post2.
The joke cuts two ways. Anime fans posted it earnestly as a cute affectation, while others copied the style ironically to sound "cute or unwitting"2. Image macros pairing Suiseiseki's face with walls of repeated "desu desu desu" text became a fixture on /b/ and /a/, and the word's Tourette-like rhythm made it easy to bomb into any thread.
How It Spread
How to Use This Meme
Desu posts typically pair a Suiseiseki image with the word repeated many times, or end an otherwise normal English sentence with "desu" as an anime affectation. A common convention on 4chan was to fill an entire reply with nothing but "desu desu desu" as a thread derailment tactic, often triggering a Gaston counter-spam in response.