Top Definition
A famale. She's a man eater. She will get into you mind and wreak havoc. She will leave you wondering WTF just happened. Avoid these woman at all costs they will eat you alive.
Example 1:

Boyfriend: Lies

Girlfriend: Know's he's Lying and plays along.

Boyfriend: Thinks he's in the clear.

Girlfriend: Give's him his morning coffee in a dirty coffee cup with laxatives in it!

Boyfriend: Just got hit by the mental ninja!

example 2:

Bob: Dude what's wrong?

John: (insert name) is fucking with my emotions, She's like a mental ninja.
by Snickelfritz33 February 02, 2010

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