Janusz

Apparently originally a diminutive of Jan which was used as an independent given name from the Middle Ages onwards: https://pl.wikipedia.org/wiki/Janusz [noted -ed]

Jan and Janusz may sound similar even to Polish speakers, but I think they have different roots. I see much more similarity of this name with the Latin Janus rather than the English John. Another example could be Marius (Latin name) and its Polish equivalent Mariusz. In fact, all Roman male names could be made to sound Polish just by adding a "z" at the end.

Formerly it was a diminutive form of Jan (John), later it developed into a separate name.