This refers to certain characters sometimes found in fanwork (especially [[Touhou]]-related). They are usually naked and always depicted wearing bags over their faces, usually marked with 罪 ([i]Tsumi[/i], or "Sin"). As a result, they are referred to as 罪袋/[i]Tsumi-fukuro[/i], which literally means "sin bag" or "sin sack".

While whom they specifically represent is never specified, it seems a safe bet to liken them to the more eccentric 2ch users and otaku because of their tongue-in-cheek speech and unwavering fanaticism and lust for the characters with whom they're depicted.

You'll most frequently see them depicted as loyal (and often abused) fans and followers of [[Yakumo Yukari]], most notably in the promotional video for the Coolier song "Yukarin Fantasia" (one of their earliest appearances, if not THE earliest) and the IOSYS promotional video for "Border of Extacy".

They are sometimes described in English as "[[Anon]]", referring to the collective anonymous members of the image board 4chan. Though technically not the same thing, the two concepts are somewhat similar.
罪袋 農袋