nekoguchi 猫口

In anime, sometimes a character's mouth is drawn like the number three rotated, like this: (:3) or this: (´・ω・`). The term for that in Japanese is nekoguchi 猫口, "cat mouth."

Left: Pipimi ピピ美
Right: Popuko ポプ子
Anime: Pop Team Epic, Poputepipikku ポプテピピック (Episode 1)

• Meaning
• Examples
• ケツみてぇな口しやがって
• References

Meaning

The word nekoguchi 猫口 is a noun compound formed by neko 猫, "cat," and kuchi 口, "mouth." The ku く becomes gu ぐ due to rendaku 連濁.

Naturally, it can refer to an actual cat's mouth, but in anime, it refers to the cat-mouth-like mouth drawn in anime characters.

Character: Neko ネコ
Anime: K (Episode 4)

For the record, although some instances of nekoguchi somewhat resemble a duck face (the photo pose), they're different things. The word ahiruguchi アヒル口, "duck mouth," is how you say "duck face" in Japanese.

• See: アヒル口はもう古い！ハリウッドで有名な魚口って知ってる？ - youtube.com, accessed 2020-02-03, for reference.

The term nekoguchi refers to mouths that look like 3's ROTATED. The following is not nekoguchi:

Character: Kuga Yuuma 空閑遊真
Anime: World Trigger, ワールドトリガー (Episode 16)

• me ga san
  目が３
  The eyes are 3's.
• kuchi ga san
  口が３
  The mouth is a 3.

Examples

For reference, some examples of nekoguchi 猫口.

Anime: Boku no Tonari ni Ankoku Hakaishin ga Imasu. ぼくのとなりに暗黒破壊神がいます。 (Episode 1)

• （＾ω＾≡＾ω＾ ）おっ

Character: Churuya ちゅるや
Anime: Nyorōn☆Churuya-san にょろーん☆ちゅるやさん (Episode 1)

• にょろーん（• ω •｀）

Character: Kanzaki Hideri 神崎ひでり
Anime: Blend S, ブレンド・S (Episode 8)

• （ㅎ ω ㅎ）

Character: Nanachi ナナチ
Anime: Made in Abyss, メイドインアビス (Episode 10)

• （=ㅎ ω ㅎ=）

Character: Kurumizawa Satanichia McDowell 胡桃沢=サタニキア=マクドウェル
Anime: Gabriel DropOut, ガヴリールドロップアウト (Episode 4)

• （´＞ ω ＜`）

Sometimes, a character's face turns into a cat face for some reason. When this happens, nekoguchi is sometimes accompanied by "cat eyes," nekome 猫目.

Character: Misaka Mikoto 御坂美琴
Anime: Toaru Kagaku no Railgun, とある科学の超電磁砲 (Episode 7)

• （ ⓛ ω ⓛ ）

ケツみてぇな口しやがって

Because the shape of nekoguchi sort of resembles a butt, it's become a meme to say stuff like this:

Character: Izumi Konata 泉こなた
Anime: Lucky☆Star, らき☆すた (Episode 1)

• ketsu mitee na kuchi shiyagatte
  ケツみてぇな口しやがって
  [You keep going around with that] mouth that looks like an ass!
  • {ketsu mitai na} kuchi
    ケツみたいな口
    A mouth [that] {looks like an ass}.
  • shiyagatte
    しやがって
    The ren'youkei 連用形 form of suru する, plus the jodoushi 助動詞 yagaru やがる in the te-form.
  • suru
    する
    To do.
    To make something become somehow.
    In this case, to make your mouth look like something. To have a mouth shaped like something.
  • ~yagaru
    ～やがる
    To have the nerve to. To dare to. Used to imply someone is doing something impudent.

The origin of this meme appears to be a very random NSFW comic posted on Pixiv.

References

• 猫口 - dic.pixiv.net, accessed 2020-02-03.