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1.
To double team someone penetrating the anus, and the mouth, or on a women, the vagina. The origin is a plan during World War I in which the Germans had to fight war on two fronts, or attack two fronts.
My friend and I are going to Schlieffen you until our penis's meet.
by eo2340 June 24, 2014
 
2.
(verb) "to schlieffen" to attempt to enter into someplace or something, such as a room or a bodily orifice, without warning and unnoticed until the last minute
(noun) "a schlieffening" an attempted entrance into a place or thing, such as a room or bodily orifice, without warning and unnoticed until the last minute

Etymology: from the German Army's battle plan for World War I, the "Schlieffen Plan," characterized as a "massive, unexpected, and successful surprise attack."
Richard: "Last night I schlieffened my wang into Rachel's ass, and once the initial surprise was over, I think she thoroughly enjoyed it."

Hank: "Did you hear how Richard executed a successful anal schlieffen with his girlfriend? It inspires me to try one myself!"

Homer: "I was late for class, so I schlieffened into the room while Mr. Jones' back was turned, but alas, I was caught."
by yargenhargen July 03, 2014
 
3.
This word can be used as a substitution of "stuff" or a less vulgar term for "shit". It is scientifically proven that 87% of Americans find schlieffen to be more "fun" to say". Also, "Schlieffentag".
Now that's just a load o' schlieffen.

AH KAN' BUH-LEEEEE' DAT SCHLIEFFEN!
by John Foppe August 14, 2003