"Douman Seiman" is a line from the refrain of the song "Let's Go! Onmyouji" from the Power Instinct: Matrimelee soundtrack.
A CG-animated music video of this song was produced by the game company. Shortly thereafter, it was leaked to the Internet - NicoNico, in particular - and from there it became a popular meme. Memetic mutation set in (e.g. Yabeno Hikomaro's line, "Kuyokuyo suru na!" ("Don't brood on it!") was mondegreened as "Puyopuyo suru na!" ("Don't get soft and squishy!"))
Several other videos and songs have made reference to this video. One of the most notable is "Tsurupettan" by Silver Forest, which uses Kotohime's first sung part, modified slightly, as a bridge into the refrain. (It also used the "puyopuyo suru na!" meme above.)
