shingesu 真ゲス

In Japanese, shingesu 真ゲス meaning literally "true scum," refers to the gesugao ゲス顔, "scum face," of Shingetsu Rei 真月零, a certain character from Yu-gi-oh Zexal 遊☆戯☆王ZEXAL.

Anime: Yu☆Gi☆Oh! Zexal 遊☆戯☆王ZEXAL (Episode 96)

Definition

In anime, sometimes evil, scheming characters scheme evils, and that shows in their smug, scummy expression. In Japanese, that facial expression is called gesugao.

Manga: Death Note (Chapter 53, 悲鳴)

keikaku doori 計画通り
All according to the keikaku.
- Note: keikaku means "plan."

In Shingetsu's case, his gesugao was so extreme that it was given its own name: shingesu.

The term shingesu 真ゲス is a portmanteau from his name, Shingetsu 真月, and the word "scum," gesu ゲス, which we can assume was actually taken from gesugao. So shingesu doesn't actually mean "true scum," it just comes from the character's name.

However, it's a trope to call a something that's in full-power a "true something," like one's "true power," or the "True Goddess Reincarnation," Shin Megami Tensei 真・女神転生, and stuff like that.

Therefore, shingesu kind of also ends up having that meaning of a true gesugao, a gesugao that's like Dracula, the true vampire, much more powerful than the weaker imitations.

Meme

Although shingesu was named after the Zexal character, it's become a meme, so you may see it in illustrations that don't feature the Shingetsu, or even Zexal at all.

こんな夜神月はすぐ捕まる

In fact, you can even find references to this meme in manga and anime.

$Runrun ルンルン displaying a shingesu 真ゲス style gesugao ゲス顔 for a fraction of a second was an Easter egg hidden in the anime.$