So Dixon for the second time in two minutes finds himself laughing without the Motrix of honest Mirth,— this time, a Mr. Mason-how-you-do-go-on laugh, sidewise and forbearing, the laugh of a hired Foil.
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Line 123 Vulgarized:
Dixon fakes a laugh for the second time tonight. The first time, Dixon, confused by Mason's invitation to a drop at Tyburn, laughed uncomfortably. This laugh, more precisely executed: Dixon looking off to the side, maybe with a wave of one hand, his beer cradled close to his chest with the other, a languid, "Ah ha ha ha," with no power behind it, an obvious forgery though not intended to patronize.
Subtext:
People lie. They lie in many varied ways. In small ways not intended to harm, or misrepresent but to improve sociability, to keep peace. At the smallest scale, one of two people interacting over drinks, we find so many little falsehoods made for a variety of reasons. Now expand this out. To a household. To a large committee. To a community. To a village. To a nation. To the history of the world. The lies, the untruths, building, piling up, gathering steam, overwhelming anything that could possibly be called Truth. This is history. A Jenga tower formed half-full or more with blocks of lies and no way to identify them from the truth. Because hi(stories) pass through multiple tellers, move into the future, are finally written down long past the point where the people who lived them can tell them true. And, again, even if they could tell them true, they're still from one point of view, and that view can't know what the others were thinking, when they were lying, and why.
The point? History is what lies and truths combined have been accepted. Pynchon isn't writing history here (nor is Reverend Cherrycoke telling a true history for future generations; he's trying to stay warm and fed by engaging children); Pynchon is writing a novel with historical figures. And novels often tell actual true things because they aren't mired by the need for actual facts. They tell truths about human nature, about the need for empathy, the terror of brief existence, the responsibility of community, and the weight of desire.
Sometimes we're told lies and we react truthfully. Sometimes we're told truths and we lie in our reactions. And then there are, mathematically, some other ways to arrange those things as well!
Or maybe Dixon just fake laughed like a posh guy at a cocktail party in a black and white movie from the '50s and there wasn't meant to be any more to it. It's Pynchon! Who can tell?! He may have just wanted to reference more modern and specific media here! Like when Slothrop sees a newspaper spin around in the sky or turns into a comic book superhero.