A druggie, a not sane person, an idiot, a person who punishes another person unfairly
Teacher: Go to the office
Student: Thats not fair, you're a tweaker!
Tommy: Hey, Joe, look at John today, he's like high or something!
Joe: Man, he must be a tweaker!
Crazy guy: Look at the flying donuts!
Insane asylum guard: My god, he is crazy!
Insane asylum guard 2: Man, he's a tweaker!