I think it started as a chapotraphouse joke. But it really is just the literal meanings of the words. He's tall and fat (big), constantly glistening with sweat (wet), male and childish (boy).
I'm normally too afraid to ask about inside jokes unless I see them come up like a lot and still can't extrapolate the meaning behind them. I usually understand them after a while, if not immediately if they are readily apparent. But this one just kept eluding me. I just wasn't sure if it was meant to be taken at face value or if it had some in-joke reasoning.
JFK was famously so dry that the CIA had to unalive him at Dow Chemical's request so that they could research his body to invent those little silica gel packs.