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

.