"Wah! What are we gonna do on the bed?"
The onomatopoeia for someone (usually a little girl) falling backwards onto a bed. The meme originated in a panel (post #2301100) from the loli doujin Tsukimisou no Akari (The Light of Tsukimi Manor 2).
Use this tag when a post uses the word "pomf" as a sound effect. For the ordinary "puff of air" effect, use =3.