1) A speed, or meth addict who has assumed the role of the drug, in other words, they lost their personality to the drug.
2) An ex- deadhead or hippie who has fried their brain and is now living in the small, paranoid world of warped memories of a mundane adolescence. They communicate by spreading urban legends as "amazing" inspirational stories that align with their perception of reality.
3) A stupid, spastic teen (or adult, but less common) that display ignorance and naivety with every action.
I was walking through the Haight the other day and was surrounded by tweakers.
I went to my high school reunion and the tweakers were still messed up in the head.