Every Monday we will post an entry that hasn’t yet been published with a view towards harnessing the collective onomastic power of the internet. If you have any thoughts about the name’s origin, other variants it might be related to, other examples of its use, etc., please share them in the comments! If you wish to browse other Mystery Monday names, there is an index.
12th-century France is full of lovely, unusual feminine names, and one of them is today’s Mystery Monday name.
There is a Latin explanation for what it might be: fisa is a perfect passive participle of Latin fido ‘trust, have confidence (in)’. It is possible that this is the root of the name, but it would certainly be an unusual construction.
Does anyone have any thoughts? Any alternative explanations (probably, Germanic)? Please share in the comments!
Dictionary of Medieval Names from European Sources 
Looking up Förstemann (1900) there is no Germanic candidate name to explain the name form Fisa. So a derivation from the Latin name Fides “Faith” borne by two saints (St. Faith of Conques and Sts. Faith, Hope, and Charity) is the most probable direction to go: The d gets deleted and for clarity a clearly feminine ending -a was added