Real-Life Weirdness

[Nereidalbel]

Originally Posted by William

The Joker Grin isn't entirely fictional: there are several ways to produce a smile-like rictus in a murder victim, including strychnine and a certain poisonous flower.

Originally Posted by Flyndaran

It's really just a hideous grimace vaguely reminiscent of grinning. Also all the other horrific symptoms coinciding with it kind of render the grin rather minor in comparison.
But as The Joker was a genius chemist in some iterations, it makes sense that he could have developed a poison whose primary symptoms before death was risus sardonicus and paroxysms reminiscent of laughing.

The ingredients listed in comics canon actually get you the right symptoms. As pointed out in the video, Joker's genius-level chemistry helps with some of the mixing issues one would normally encounter.