The word sayzin comes from some dudes nickname or pull off of a nickname. We use it as a general use word. It could mean a dumbass, a fucker, a police officer, whatever you really want to imply with it. A filler word if you want to look at it like that, or a replacement word.
Why did you do that you sayzin.
Ronnie is such a sayzin for putting up with that girls bullshit.
Hey, watch out for that sayzin right there, he's clocking people with his headlights off... you get the idea.