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.
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).