In chat, it is the act of saying something before someone else. It only counts if someone else actually says the same thing afterwards.

When posting on a forum, you submit a post only to find that someone has posted the same thing only seconds earlier.

(V): When someone gives another person such a good comeback that the person being exposed to the torment never even sees it coming. But the burn does not end there my amigos. The Ninja'er comes back for a second attack, ending with their battle cry, "Ninja'd!" Then disappears before you even have time to pee your pants from shock. Here's a diaper baby, clean yo'self up. You be dirtying up my new kitchen floors. .. bitch.

the act of doing something successfully in a stealthy manner & when exiting the scene, you say "Ninja" in a spooky tone to rattle other people in the area who didn't realize wtf was going on.

v. the act of stealing, swooping, or snaking something. the person who has been ninja'd upon does not realize it for a little while.

When a Nerd playing World of Warcraft, steals an item that he has no apparent use for and his group member needed it.

When someone takes something or someone away from you and you did not know whats the fuck is going on until the person ninja'ding you is laughing his/her/ ass off. The term ninja'd is usually mixed up with thiefed but the stupid sob that did that is spread across the atlantic ocean....Right now.