The pigs aren't always very grammatically correct, but I feel like "for thy" is possibly just a fragment of some longer phrase they're meant to say, like "for thy plop picking" which I'm pretty sure I've seen them say before.
Steps to Reproduce
pick up manure around the town, some of the pigs just say "for thy" sometimes when paying you for it
pick up manure around the town, some of the pigs just say "for thy" sometimes when paying you for it