Lexicon Balatronicum (1811) cites caster ‘a cloak’ as Cant and possibly has a connection with this. Smart & Crofton (1875) collected the form Plaáshtan with the same meaning from English Gypsies, they also collected forms Póktan, Poxtan meaning ‘cloth’ from the same source.
WG further suggests that it could be from Gaelic plaide, itself from Scots plaidie.