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1.
A condition marked by frequently startling people on accident, such as while walking up behind or beside them to say, "Hello." Most accurately described as pervasive accidental sidling.

Ninjavitis most commonly occurs among those who wear wrestling shoes, women with small feet, and the Japanese.
Person 1: (Casually walks up beside Person 2) Hey, man!
Person 2: Dude, you scared the crap outta me! It's like you've got ninjavitis.

See also: sidler, sidle
by The Cruz October 17, 2010
 
2.
1. An oral condition that causes ones breath to be so noxious that it can sneak up on unsuspecting people and be nearly fatal.
2. When the physical state of ones mouth is so repugnant that it looks like a skilled ninja has drop-kicked them squarely in the mouth.
1. I heard that John is in the hospital, cuz he went to kiss Sheila goodnight, and her ninjavitis made him go into shock.
2. god rest his soul, but ODB musta been in a fight with the RZA cause even his goldfronts had sum ninjavitis
by deek da biker January 17, 2006
 
3.
When someone has severe memory loss due to excess consumption of alcohol, one is said to have Ninjavitis.
"I can't remember anything about last night, had a serious case of Ninjavitis"

"I can't believe I got with that ugly bird last night, Ninjavitis strikes again"

"Mate where did everyone go last night? Oh Ninjavitis"
by cass254 August 25, 2009