A further bastardisation of the word "pure" by scousers.
"Yer ma's a pyay slag, lid!"
a colloquiaism from Liverpol that replaces 'very' or 'really'. It has evolved from Liverpudlians using the slang term 'pure' to mean the same thing, it then morphed to become pyar and then pyay.
"It's pyay hot in here"
"That exam was pyay hard"