A G.I. Joe, man
The ‘G.I. Joe’ is a reference to the line before and after. G.I. Joe was a cartoon show and also recently a movie about a group of super soldiers who defend America (reference to the ‘soldier’ in the previous line). The movie was called ‘G.I. Joe: The Rise of Cobra’ which references the line before about “making her ass spread like the back of a cobra’s”. A G.I. is also a general term for an American Soldier. He’s “down there” in the titties like a solider is down there in the battle field. And he’s about to “go in” — i.e. about to parachute down into the battlefield, or in this case her titties.
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