So unfashionable it is awesome.
Alternate definition: Appreciation or disgust for someone so completely out of tune with societal norms, taste, or taboos. So horribly unfashionable it is like... wow.
Wearing a tropical shirt and shorts in the middle of winter can be plab.
"you are so plab!!!"
"betsy is so mean!!"
"i know she is unplab!!!"
"Oh my gosh, you are so plexy"