A hybrid of the phrase's filthy ninja, and filthy seagull (see definitions).

To perform a Filthy Ninja Seagull, you need cunning and the agility of a chimp(and male genitals).
The act of Filthy Ninja Seagull, is to (like a filthy ninja) sneak into a room of a couple humping without bein seen or heard, climb on to the nearest wardrobe or chest of draws. Once this is done whop out your member and proceed to masturbate. On reaching climax(providing you've not been caught) start to screech like a demented seagull whilst spraying your man muck preferably over the couple whilst they're still at it. This now is where you need to be quicker than a leopard, and swifter than a er......... swift. Before the couple realise what has just taken place, or turn a light on you need to, to put it plainly, get the f@*k outta there without being seen. Thus leaving them wondering how the bloody hell did a bloody seagull get into the room.
To perform this act successfully will instantly make you a LEGEND.
Example 1:
Master '' you have done well young grasshopper, you have successfully completed the Filthy Ninja Seagull task''.
Grasshopper "Thank you Master"
Master " However next time try using another couple other than your parents"
Example 2:
As the door closes and the squark of the seagull slowly fades away, Mike turns to Carol, both covered in the sneaky birds mess, and asks "how the hell did a seagull get in here,it was like a bloody ninja"
by the real sweet-a-bix July 5, 2010
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A hybrid of "Filthy" ninja and "filthy seagull".
The act of sneaking in on a couple having sex without, being seen or heard, then climb onto either a chest of draws or wardrobe and proceed to masterbate furiously to climax. On spraying your man muck, squark like a seagull, and attempt to leave before a light is turned on or the couple get up to beat your ass.

A dangerous and tricky operation, but when done right turns the trainee ninja seagull into a living legend!!!
teacher says to student " Well done grass hopper, you have successfully completed your filthy ninja seagull mission. However extra points have been awarded as it was your parents havin sex"

"Where the fuck did that seagull come from?" said mike to Carol
by the real sweet-a-bix July 3, 2010
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