This expression came from the H-game (PC)/PS2/DC/anime [Kanon]. Ayu Tsukimiya's line (and probably the most famous throughout Kanon). This is her expression when she moans, or whines (like eeeehh, but more original).\r
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Darn though. She had a sad story. One of my few favorites besides Nayuki.
> Ayu: I'm cooking cookies.
> Yuuichi: I bet they'll taste terrible.
> Ayu: Uguu! I've learned how to bake from Akiko-san!
> Yuuichi: You can't learn how to bake with just watching in a day..
> ..although I'll eat them if you manage to make them look good.
> Ayu: Really?! Then I'll do my best!