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.
How about a very strange name from 13th century Latvia? In the Rigische Schuldbuch (1286-1352), a man named “Yuwage” was recorded in 1290.
A lot of the names in this register are ordinary German names, easily recognisable underneath their Latinization. But this is an exciting source precisely because so many of the names in it are not ordinary German names, or are significantly masked by their Latinization — and this name is one of them.
We have no idea, not even a guess, about what the underlying name is. We’d love to know if you have any thoughts! Please share them in the comments.
Dictionary of Medieval Names from European Sources 
There is an analysis of the names in that source on pages XXXVII ss. On p. XLIV this one is grouped with the Livonian names, with a note “Yuwage (juwa im Livischen = wohl, recht)”.
https://books.google.com/books?id=RGdSAAAAcAAJ&pg=PR44
It is at least in the right geographical area for a Livonian name. The source’s juwa is probably the jõvā of Jõvā pǟva! ‘Good day!’ in the Wikipedia article at the link above. I’d guess that the -ge of the name is a variant of the Low German diminutive suffix -ke.