One of those ludicrously sticky and sweet liquors that exist on top shelves in every bar and which every bartender knows full well no-one will ever drink. Death may have had one when he tried to drink everything to dissipate his sorrows in Mort.
In Lords and Ladies, one of the young would-be witches in the coven run by Lucy Tockley calls herself Amanita DeVice, much as Lucy prefers being called Diamanda. There does seem to be a sinister cachet to the name.
On Roundworld, Amanita is a genus of mushrooms responsible for 95% of deaths from mushroom poisoning. There are a few edible species, some common as food where they are native, but in general it's best not to eat any member of the genus.