Historically, it meant a Japanese spy or special agent, but now it's more famous for its image as a mysterious assassin in pop culture, sometimes symbolizing a character with superhuman abilities.

It became world-famous thanks to the manga 'Naruto.' You can use it wittily to mean 'Appeared like a ninja!' when you've completed a difficult task in secret or shown up quietly at a meeting spot. It's the perfect emoji to emphasize speed and stealth.