Jaisu - nickname for Polish equivalent of John?

Greetings,

A library customer has the impression that Jaisu is a nickname for a
Polish "equivalent" of John, Jan maybe. But unlike Janek, Jaisu
doesn't look anything like Jan. Is there a story about how Jaisu came
to be a nickname for Jan?

Does "Jaisu" have some meaning or use in Polish apart from a nickname
for John? Our two Polish dictionaries and the online resources I've
checked have not been helpful in this regard.

Any information or suggestions about who else I might ask will be
appreciated.

Tom Zimoski
Fresno, Calif. USA

silvagen...

"Jasio" is indeed a nickname for "Jan", the Polish equivalent of "John".
There is no story behind it -- there are many Polish nicknames ending
with -io (Stasio/Stanislaw, Tadzio/Tadeusz, Józio/Józef, Stefcio/Stefan,
Rysio/Ryszard, etc.).

The form "Jasiu" (not "Jaisu", which is a typo or a misspelling) is
actually the vocative declension case of "Jasio", not the nominative.
There have been an increasing tendency in Polish to use the vocative
case instead of the nominative for the diminutives of male given names.
Such usage is still regarded as incorrect, but it's gaining ground. This
tendency is explained by certain features of pronunciation of the vowel
"u" as opposed to "o" and the pronunciation of palatal consonants. (To
put it simply: it's slopiness.)

-Margaret Mikulska
silvagen@gmail.com