〆

In Japanese, 〆 means shime 締め, "fastening," "tightening," the noun form of the ichidan verb shimeru 締める, "to fasten," "to tighten." To elaborate: this weird-looking symbol is just a quick way to hand-write shime.

Manga: Gekkan Shoujo Nozaki-kun 月刊少女野崎くん (Volume 1, Chapter 2, Page 18, ケガの理由)

• Kanji
• Usage

Kanji

Believe it or not, 〆is technically a kanji 漢字, "Chinese character," except it's a "Chinese character made-in-Japan," wasei-kanji 和製漢字, which, as far as I'm concerned, is the shiny variant of wasei-eigo 和製英語.

Usage

The shime 〆 simply replaces shime 締め wherever it's used.

• shimeru
  締める
  To tighten. To faster.
• shimeru
  〆る
  (same meaning.)

Manga: Gekkan Shoujo Nozaki-kun 月刊少女野崎くん (Volume 1, Chapter 2, Page 18, ケガの理由)

• Context: Sakura Chiyo 佐倉千代 sees Nozaki-kun, who is a mangaka 漫画家, exhausted, guesses why.
• aa
  あぁ
  Aah.
• shimekiri ka...
  〆切か･･･
  Deadline, huh...
  • shime-kiri
    締め切り
    Deadline. As in the time limit to deliver a completed work, such as a completed manga chapter.