What you use when you are trying to insinuate something and they pick up on it. It's like a get out of free jail card for those moments.
Ashley: Maybe you shouldn't wear that dress.
Lucy: Why not?
Ashley: It doesn't fit right.
Lucy: Are you trying to say I'm fat?
Ashley: I'm not sayin', I'm just sayin'.
Trying to let someone thing you're "not sayin'", when, in fact, you most certainly ARE "sayin'".
Trying to make it seem like you are not saying anything unkind or questionable, but then admitting that you are. It happens so fast, no one will be the wiser...
Beezus : Does this pair of jeggings make my ass look enormous ?
Ramona: Well, maybe a little.
Beezus : Dang - for reals ?
Ramona: Hey, I'm not sayin', I'm just sayin'
Basically means "I'm not implying anything, I'm just making an observation". Which of course doesn't mean that the person *isn't* in fact saying, i.e. implying.
Mitt Romney sure made a 180
on national healthcare... I'm not sayin', I'm just sayin'.