Difficult and rare words/expressions that never show up in vocabulary lists

Solution 1:

This is slang from the 19th century that is no longer used much anymore. Mark Twain probably heard these expressions frequently, as I lay that they were in more common use back then. Some of them are going to stump native English speakers today, as well. I believe I have only seen "truck" used in that sense in books.

For "lay", "I lay" = "I bet". My guess is it comes from "lay a bet on".

Solution 2:

Is it just a matter of reading a lot and attempting to infer it from the context?

Yes. That, or using a dictionary. For example:

truck 2 |trək| noun
1 archaic barter.
• chiefly historical the payment of workers in kind or with vouchers rather than money.

That's where Twain's usage comes from. In this sense it means something like to get involved or negotiate with someone or something.

When I read Tom Sawyer and Huckleberry Finn I was in the 5th grade. The usages were odd then, and my grade-school dictionary didn't have those words. But I could get the sense of what was being said, even if I didn't have precise definitions for all the words, and before long I didn't even need to look up the words because I understood them from context.

This is a key notion to understanding another language anyway. You have to get to the point where you can understand what someone is telling you without knowing all the vocabulary. Some of it you have to look up, though, as you did when you learned eleemosynary — perhaps from reading Fielding's Tom Jones, I believe in his description of Squire Allworthy? That too is archaic, but easier to find because it is not a vernacular usage of a common term.