It's basically the opposite of 'yas'. Like every person is gonna use it to express their disguist, dislike.

Like it's gross, pathetic or you just don't like it.
Obviously it's the short form of 'yas' (the word from the gay culture and Gaga's fans that every straight girl says) + NOT.
-OMG HAVE YOU SEEN EMMA'S NEW BOYFRIEND?
-YES GIRL. IT'S soOOooOo YASN'T! EW!
by Sebii May 11, 2017
Get the Yasn't mug.