A portmanteau of "trashy", "slut" and "fashionista". The word "slut" is omitted as it can't fit in fashionably.
It is used to denote a trashy whore, slut, skank, and occasional MILF who happens to dress fashionably like a supermodel. They are easily identified by their trendy clothes and willingness to work something out with the same sex in case two are not enough.
Sara: Hey, stranger. What say you and me in the stall two doors down? Rawr.
Mark: Eh...I have a girlfriend.
Lexi: Mark! Why is that trashionista putting her hands on your junk?!
Sara: O, are you and him together? I'm sure we can work something out.