A person who has smoked enough sherm to acquire serious brain damage. This does not take long to happen, and is often characterized by trembling, drooling from a slackly held-open mouth along with a rambling, mostly incoherent pattern of speach.
Oh man, that Reggie, he ain't nothing but a shermhead now; he ain't no damn good for nothing! Alls he is is a twitching drooling fool slinking around in the alleys. He melted his brains real good.