While trying to open a beerbottle at a party, you notice there are no bottle openers. So you use something at the host's house to open the bottle while breaking something in the process.
Phil: Hey Brit how did you open that bottle?
Brit: I just ninja opened it (ninja opening) on Christina's fence, i totally chipped a piece of the fence off.
A hangover that is not evident upon waking up in the morning, but dominates your life two hours later.
At sunrise Joel bounced out of bed, gathered a crew up for brunch, sat down at the table, and proceeded to have his day destroyed by a ninja hangover that he had been harboring all along.
A stealthy ninja who poops in the toilet while someone else is using the shower. The ninja pooper then leaves it there with the lid open and exits without making a sound. They are now perplexed at whether they forgot to flush or even went poop at all.
While someone is using the shower, a ninja pooper silently walks in and takes the browns to the superbowl without them knowing. You then proceed to exit the room without flushing to confuse the other person as to whether they forgot to flush or even went poop at all. Priceless.