"Pay it, aye, thought Theon. Pay it with false coin. Jon Snow would see through the impostesure at once. Lord Stark's sullen bastard had known Jeyne Poole, and he had always been fond of his little half-sister Arya".
Could anyone help me to figure out what "impostesure" means?
"Impostesure" seems to be an invented word. In this context, it's roughly synonymous with masquerade, fraud, or misrepresentation. It's a noun that encompasses the whole false narrative that Jayne Poole is Arya Stark.