Contracion of 'say not' used as a modal verb.
Ezekiel: Howdy, Luca! How are you doing?
Luca: Hey, I'm pretty good. What's new?
Ezekiel: I came across Melinda yesterday and we had a chat...
Luca: Really? what told she you?
Luca: I ain't gonna tell you. You know that I sayn't what I am told.
by Ezechiel January 16, 2007
Get the sayn't mug.