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an internal source of extreme amounts of energy used during times of physical exertion such as sex or rowing
Idiot: Barney, it looks like you are getting a little round around the middle *poke*.
Barney: No, you idiot, that is my fuel tank. It is how I pwn you so hard when it comes to fucking hot girls and on the water!

The Fuel Tank 4 is ultra fast, steamrolling past the competition.
by Fuel Tank 4 April 18, 2006
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After having anal sex and ejaculating, the male keeps his penis inside the female's rectum until it gets flacid, then he proceeds to fill the ass with urine.
She said, "Fill me up!" so I fuel-tanked the bitch.

Since it's hard to pee when your dick's hard, you'll have to give it a moment to soften up before a good fuel-tanking session can take place.
by Robert Arthur Gilbert December 04, 2007
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