I was totally amused by this letter from Mozart to his cousin Bäsle, and like many other people I pondered the meaning of "Spuni Cuni fait." I found an answer in a Mozart Forum discussion that is no longer on line, but it's fortunate that I remember the gist of it:
"Spuni" means "spun," "Cuni" is like "Cony," which means rabbit, and "fait," is the past tense of the French verb "faire." Therefore Spuni Cuni fait means (with capitalized nouns) would mean something that was spun from rabbit hair, like the shawl that Mozart's cousin Maria Anna Thekla Mozart (i.e. cousin Bäsle) is wearing in this drawing she made of herself here:
It could actually be the Spuni Cuni fait!
You can read more of the Bäsle letters (in German with a racy Mozart drawing on a facsimile included) here.