This name is a meme in Poland. https://pl.wikipedia.org/wiki/Janusz_(okre%C5%9Blenie) Janusz - a pejorative term for a person displaying stereotypical negative characteristics of Poles. He is usually depicted as a middle-aged man with a mustache, usually dressed in a white T-shirt (usually with straps), wearing white socks and sandals, with a large belly, often holding a discount store bag. Janusz's favorite pastimes are drinking beer and watching TV. A typical janusz is often depicted with the head of a monkey (Sunda proboscis monkey).
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.
Janusz - a pejorative term for a person displaying stereotypical negative characteristics of Poles. He is usually depicted as a middle-aged man with a mustache, usually dressed in a white T-shirt (usually with straps), wearing white socks and sandals, with a large belly, often holding a discount store bag. Janusz's favorite pastimes are drinking beer and watching TV. A typical janusz is often depicted with the head of a monkey (Sunda proboscis monkey).