New euphemism: Trouser Dwarf.

Knowing this artst's pervertedness I wonder why does Sukuna smile peeking out of a place like that.

Judging from a Google Translate of the artist's commentary, the Shinmyoumaru pictures from Gaoo have been a series, starting with the faceless male receiving her in the mail and then her getting increasingly attached to him.

"Say hello to my little..."