Here's the wiki on it for more info & links. I'd def tread lightly with that stuff.
Under toxicity & historical usage,it refers to gruit. Maybe you can find a link on the page. There's a link for henbane gruits at the bottom. It goes to a dictionary-more like an encyclopedia-of beer history. It also goes on to discuss how porter began as 3 ales mixed together,called "three threads. A mixture of common ale,bee,& 2 penny ale. Ralph Harwood brewed it into one beer At the Bell Brewery in London.