How come the msq keeps using big words?

[Iscah]

I guess the best way they could think of to directly translate that into English was to use things like thy/thee/thine/thou and all that?? Since English doesn’t have anything like polite forms versus casual forms the way Japanese does; and thy/thee/thine/thou was once used in English as pronouns. Though, ironically, they conveyed intimacy with the other party one was speaking to, as opposed to being used for politeness (which would imply social/personal distance between two speakers)—so, technically, they can be argued as casual forms. If one was to consider their actual use back in the day.

"You" is also the plural form of "thou" - so you'll occasionally see Urianger using it if he's addressing more than one person at the time.

https://en.wikipedia.org/wiki/Thou