What is the origin/meaning of wish't?
It looks to me like a dialectal spelling of "wish(ed?)". I found it in the songs/poems "Wish't I wuz a gurl" and (close enough) "Wush't I wuz a boy" (example in Health). Here's an earlier example from 1822: "I wish't, I want it".
I found "wysht" in the MED as a spelling for the past tense, which might be related or not. (That's really not different from "spelt".)