Scottish, probably a combination of John and Ann. While it is a common practice in European Catholic countries, combining one male name and one female name is extremely rare in Britain. We have been unable to find any reference to this name or the practice of combining English language names in this way.
Johnann, though rare, is fairly widespread in the Scottish Censuses and Parish Records in the 19th century. A correspondent [GB] notes that the examples he has found were mainly, but not exclusively, in fishing communities in north-east Scotland.
Johnan is a spelling variant and a number of examples of Johnanna/Johnana - a combination of John and Anna - have also been found.