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It was in Crash Course World History. It is at 6 minute and 12 second. Here is the context:
We talk a lot of smack about taxes, but it turns out they are pretty important to creating social orders.
I have looked it up in the Oxford and a few other dictionaries, but it seems to me that no definitions they give fit.
In this context, Smack is slang, meaning:
Make boastful or insulting remarks, especially in order to demoralize or humiliate someone.
In this case, the sentence can be rewritten as:
We make a lot of insulting remarks about tax (as a population, in general), but it turns out they are pretty important to creating social orders.
The meaning of the sentence as a whole, is that while people tend to talk badly about taxes (as often, it's from the perspective of losing money you earned), there is a social benefit to them which this podcast goes into more detail on.
smack, for example LDOCE (the first one) will show you all the different entries which contain the word smack: smack, smack of something, ..., talk smack, etc. The other dictionaries in that list offer the same feature in the sidebar (e.g., Cambridge has a More meanings of “word” section).