Chirashi no Ura (チラシの裏, lit. “flyer's backside”), or ChiraUra (チラ裏), refers to how a flyer's backside is usually blank and may be used for doodling or writing notes. However, fliers aren't made out of quality material and double-sided fliers are more common these days.

It was popular to say “write it on the back of the flyer” when someone starts to using a 2channel thread as a personal diary for no reason and post trivial nonsense that only the poster would care about. However, it should be noted that people actually draw or write stuff down on the back of fliers too, so the context matters in some cases.

In the 1990s, Gengoroh Tagame published a gay manga titled “My Teacher” (俺の先生, Ore no Sensei) and a scene where the student and teacher agrees to lie down on the futon was turned into AA on the internet for whatever reason. Of course, the text art wouldn't be relevant to this article if the text remained the same, so it was slightly modified so the teacher was telling someone to write on the back of a leaflet.

　　　　　　r;ｧ'N;:::::::::::::,ｨ／　　　　　 >::::::::::ヽ
.　　　 　 〃　　ヽﾙ1'´　　　 　 　 ∠:::::::::::::::::i
　　 　 　 i′　　___,　-　,. = -一　 ￣ｌ:::::::::::::::ｌ
.　　　　　 ! , -＝=､´r'　　　　　　　 　 l::::::/,ﾆ.ヽ
　　　　　　ｌ　　　　　　　　_,, -‐''二ゝ　 ｌ::::l fﾞヽ |、 This isn't your personal diary,
　　　 　 　 ﾚー-- ､ヽヾﾆ-ｧ,ﾆ;＝、_　　 !:::l ） } ト
　　　　　　 ヾ¨'７"ry､｀ 　 ｰﾞ='ニ,,,｀　 　 }::ヽ(ノ 　so go write on the back of a flyer!
:ｰゝヽ､ 　 　 !´ "￣ 'l,;;;;,,,.、　　　　　　　,i:::::::ﾐ
::::::::::::::::ヽ.-‐ ﾄ、　r'_{　　 __)｀ニゝ、　 ,,iﾘ::::::::ﾐ
::::::::::::::::::::Vi／l:::V'´;ｯ`ﾆ´ｰ-ｯ-,､:::::`"::::::::::::::;ﾞ　, 　Okay?
:::::::::::::::::::::::::N. ﾞ､::::ヾ,.｀二ﾆ´∠,,.i::::::::::::::::::::/／／
:::::::::::::::::::::::::::::l　ヽ;:::::::::::::::::::::::::::::::::::::::::::／　／
::::::::::::::::::::::::::::::!　:|.＼;::::::::::::::::::::::::::::::／　／

• The kanji, “ura” (裏), means back, bottom, or other side, but this reference gets lost in translation as seen with early translations of Nijiura, so stuff like “nonsense” (бред) and “random” is used instead.
• The aforementioned manga is available with an English translation on E-Hentai and such. As for the exact scene, it can be viewed on Page 30 of the gallery.

• Chirashi no Ura ni demo Kai Tero on the 2ch AA Illustration Project
• Chirashi no Ura on Niconico Encyclopedia
• Chirashi no Ura on Pixiv Encyclopedia