As one theory goes, some wag noticed that the expression of a person whose facial muscles were paralyzed by a treatment with "busu" apparently resembled less than-appealing women he may have encountered; thus, the word "busu" eventually came to refer to people whose faces look like they were poisoned with monkshood, and then more generally to people with less-than-appealing facial appearance. (Relatedly, there is a kyogen play entitled "Busu", where monkshood is featured as a plot device. There are performances of it on YouTube, e.g. https://youtu.be/D-8cbbUPRNo)The closest analogue of this derogatory term to modern times would be "Botox face": both botulinum toxin and monkshood have paralyzing qualities, that one treated by any of those toxins in inappropriate dosages has an unnatural facial appearance, and is thus subject to mockery.(2/2)