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When somebody as an above average or otherwise huge penis when erect but shrinks to near disappearance when flaccid, or in other words vanishes like a ninja.
Example: That mother fucker has a ninja dick.
Example 2: I got ninja dick. that shit maybe 1 or 2 inches long till I get a hard on but then that shit massive.
Ashley : Brady has a total ninja dick, it was so weird!
Amanda : Oh gross no way!!!