To have silent sexual intercourse on the floor with minimal clothes removal, as to avoid discovery of fornication. Usually done by people who still live with their folks or prudish friends.
Juan: yo, we gonna have to ninja bangtoday, mamacita, cuz my folks are in the next room
To flip someone's backpack inside out, insert all the books/stuff back in it, zip it back up, and hide it somewhere. This is a great school prank to your friends.
John: Yo where's my bag?
Steve: Not sure, but Alex told me you got ninjabagged so good luck.